Antidepressant Venlafaxine During Pregnancy Linked to Heart Defects in Offspring: New Rat Study Raises Important Questions

Table of Contents

• Key Points
• Why This Research Matters: Depression, Antidepressants, and Pregnancy
• What the Researchers Wanted to Find Out
• How the Study Was Conducted
• Key Findings: Effects on Pregnancy Outcomes
• Key Findings: Heart Defects in Fetal Rats
• Key Findings: Changes in Serotonin Signaling Genes
• What This Means for Patients
• Study Limitations
• Recommendations for Patients and Healthcare Providers
• Frequently Asked Questions
• Source Information

Key Points

• In a rat study, venlafaxine exposure during pregnancy increased total fetal cardiac anomalies, including ventricular septal defects, at doses equivalent to therapeutic human doses.
• The researchers propose that increased cardiac anomalies are mediated through alterations in serotonin signaling in the placenta and fetal heart.
• This was a rat study; results cannot be directly extrapolated to humans due to species differences in drug metabolism, placental structure, and fetal development.
• Do not stop antidepressant medication abruptly; untreated depression during pregnancy carries its own significant risks for mother and baby.
• Pregnant women taking venlafaxine may be offered fetal echocardiography around 18–22 weeks to assess fetal heart structure.

Why This Research Matters: Depression, Antidepressants, and Pregnancy

Depression during pregnancy is remarkably common. Research cited in this study shows that one in every five women experiences symptoms of depression while pregnant. For many of these women, antidepressant medication is an important part of their treatment plan.

But here is a striking statistic: the rate of antidepressant exposure during pregnancy increased 3-fold in the United States between 1998 and 2005, and a staggering 16-fold increase was observed in Europe between 1997 and 2010. The most commonly prescribed antidepressants worldwide are selective serotonin reuptake inhibitors (SSRIs) and serotonin–norepinephrine reuptake inhibitors (SNRIs), which include medications like fluoxetine (Prozac), sertraline (Zoloft), and venlafaxine (Effexor XR).

These medications work by blocking the reuptake (recycling) of serotonin and/or norepinephrine in the brain, increasing the availability of these important chemical messengers. The serotonin transporter protein (SERT, also known as Slc6a4) and the norepinephrine transporter (NET, also known as Slc6a2) are the specific targets of these drugs.

According to the researchers, up to 6.2% of pregnant women with depression receive an SSRI or SNRI. This means that in the United States alone, tens of thousands of pregnancies are exposed to these medications each year.

Previous studies have produced conflicting results. Some research has linked SSRI/SNRI use during pregnancy to an increased risk of:

• Miscarriage
• Preeclampsia (a dangerous pregnancy complication involving high blood pressure)
• Preterm birth
• Decreased fetal body and head growth
• Congenital (birth) malformations
• Behavioral disorders in offspring

However, other studies have found no statistically significant effects. Adding further complexity, some of the same adverse outcomes are also seen in the children of mothers who had depression during pregnancy but did not take antidepressants. This makes it very difficult to disentangle whether the medication, the depression itself, or both are responsible for the problems observed.

What the Researchers Wanted to Find Out

Congenital heart defects are among the most serious birth defects associated with antidepressant use during pregnancy. These defects occur in the general population at a rate ranging from 4 to 50 per 1,000 live births, and they are the leading cause of infant mortality. Even when heart defects are not clinically obvious at birth, they may increase susceptibility to heart problems later in childhood and adulthood.

The researchers in this study focused specifically on venlafaxine, a highly prescribed SNRI. They noted that very few epidemiological (human population) studies have examined venlafaxine specifically, although one study did report an association between venlafaxine treatment and cardiac and neurological defects. Previous animal studies had not demonstrated an increase in malformations after venlafaxine exposure, so the research team set out to test this more rigorously.

Their hypothesis was straightforward: in utero (in the womb) exposure to venlafaxine increases the incidence of fetal heart defects and alters serotonin signaling in the placenta and fetal heart.

Why focus on serotonin signaling? Serotonin is not just a brain chemical — it is also a critical signaling molecule during embryonic and fetal development. It plays important roles in:

• Gastrulation (the early developmental process that establishes the three germ layers of the embryo)
• Establishing right/left asymmetry in the developing body
• Craniofacial (head and face) development
• Cardiac (heart) morphogenesis — the formation of the heart's structure

During early heart development, serotonin helps establish right/left asymmetry by stimulating a specific molecular pathway called the FGF8/NODAL/PITX2 pathway — a cascade of signaling molecules that tells the developing embryo which side is "left" and which is "right." This is critical because the heart is an asymmetrical organ: the left side pumps oxygenated blood to the body, and the right side pumps deoxygenated blood to the lungs. Disruption of this pathway can lead to malformations.

The placenta, too, is a major player here. It serves as an early source of serotonin that is critical for developmental programming of the fetus. The placenta expresses several components of the serotonin system, including monoamine oxidase A (MAO-A, the enzyme that breaks down serotonin), serotonin 2B receptors (5-HT2B), and the serotonin transporter (SERT).

How the Study Was Conducted

The researchers used a rat model to separate the effects of the medication from the effects of maternal depression — something that is impossible to do in human studies, since depression itself cannot ethically be assigned randomly to pregnant women.

Animals and treatment. A total of 70 timed-pregnant Sprague Dawley rats were obtained from Charles River Laboratories. Day 0 of gestation was defined as the day a sperm-positive vaginal smear was found. The rats were housed two per cage, maintained at 20°C on a 12-hour light/dark cycle, with food and water freely available.

On gestation day 8, the dams were randomly assigned to one of five groups:

• A control group (vehicle/saline only)
• Four venlafaxine treatment groups: 3, 10, 30, or 100 mg/kg/day

Venlafaxine hydrochloride (provided by Pfizer) was delivered daily by gavage (a feeding tube placed directly into the stomach) from gestation day 8 to gestation day 20, dissolved in saline at a volume of 5 mL/kg. This period covers the critical window of fetal heart development.

Measuring drug levels in the blood. To confirm that the rats were actually absorbing the medication, blood was collected from 10 dams in the control group and 10 dams in the 30 mg/kg/day treatment group. Blood samples (5 to 10 mL each) were collected by cardiac puncture, processed to obtain plasma, and analyzed using a sophisticated technique called ultrahigh performance liquid chromatography coupled with mass spectrometry. The researchers measured both venlafaxine and its major active metabolite, desvenlafaxine (also called O-desmethyl venlafaxine).

Evaluating pregnancy outcomes. On gestation day 21 (the day before normal delivery), the dams were humanely euthanized and cesarean sections were performed. The researchers recorded:

• The number of fetuses and resorptions (pregnancies that had failed early)
• Implantation sites (where embryos had attached to the uterine wall)

A total of 868 live fetuses were evaluated for sex, body weight, crown-rump length (a measure of fetal size), anogenital distance (a marker of hormonal exposure), and external morphology (overall body structure). Placental weights and diameters were also recorded.

Examining fetal hearts. After exposure to hypothermia (a method of euthanasia for fetal rats), 506 fetuses were dissected specifically to evaluate cardiac (heart) structures. The thoracic cavity was opened, the thymus was carefully removed, and the shape of the heart and major blood vessels was examined under a dissection microscope.

The hearts were then dissected using a modified Staples technique, which involves two specific incisions:

1. The first incision was made from slightly left of the apex (tip) of the heart, through the left ventricle, into the ascending aorta. This allowed evaluation of both the membranous and muscular portions of the ventricular septum (the wall dividing the heart's lower chambers) as well as the aortic and mitral valves.
2. The second incision was made from slightly right of the heart's apex, through the right ventricle, into the pulmonary trunk. This allowed examination of the ventricular septum, pulmonary valve, and tricuspid valve.

Using this approach, the researchers could identify specific heart defects including ventricular septal defects (VSDs) — holes in the wall between the heart's lower chambers, which is the most common type of congenital heart defect.

Gene expression analysis. Six placentas (three from male and three from female fetuses) and two hearts (one male and one female) from each litter were collected and frozen for molecular analysis. RNA was extracted from the tissue samples, and a technique called reverse transcription quantitative polymerase chain reaction (RT-qPCR) was used to measure the expression (activity) of specific genes. The researchers measured genes involved in serotonin signaling, including:

• SERT/Slc6a4 — the serotonin transporter
• NET/Slc6a2 — the norepinephrine transporter
• MAO-A/Maoa — the enzyme that degrades serotonin
• Htr2b — the serotonin 2B receptor
• Fgf8, Nodal, and Pitx2 — components of the left/right signaling pathway

Reference genes (housekeeping genes like Hprt1, Gapdh, and beta-actin) were used to normalize the results, ensuring the measurements were accurate.

Protein analysis. Western blotting was used to measure actual SERT protein levels (not just gene expression) in both placental and fetal heart tissues. Protein samples were separated by gel electrophoresis, transferred to membranes, and probed with a specific antibody that recognizes SERT.

Statistical analysis. The data were analyzed using one-way analysis of variance (ANOVA), followed by Dunnett's multiple comparison test when the ANOVA showed significant differences. A p-value of ≤0.05 was considered statistically significant. This is the standard threshold used in medical research, meaning there is less than a 5% probability that the results occurred by chance.

Key Findings: Effects on Pregnancy Outcomes

Maternal toxicity at the highest dose. The highest dose of venlafaxine (100 mg/kg/day) was found to be maternally toxic. Alarmingly, 35.7% of the dams in this group died during the treatment period. The dams that survived showed a significant reduction in weight gain during pregnancy: they gained only 94.7 ± 6.29 grams compared to 119.8 ± 3.3 grams in the control group — a difference that was statistically significant (p<0.01).

No effects on fetal survival or basic development. In all the lower dose groups (3, 10, and 30 mg/kg/day), venlafaxine had no effect on the numbers of implantation sites, resorptions, or live fetuses compared to control. The sex ratios of litters and anogenital distances (a measure of normal hormonal development) were also unaffected at any dose tested.

Fetal growth effects only at the toxic dose. Fetal body weights were reduced by 9% and fetal lengths by 4% in the litters exposed to 100 mg/kg/day venlafaxine. Growth was not affected in any of the lower dose groups.

Placental changes at lower doses. Although fetal growth was normal at lower doses, the placenta was affected. Placental weights were significantly decreased in the 3 mg/kg/day and 10 mg/kg/day treatment groups compared to control. The placental index — the ratio of fetal body weight to placental weight — was significantly increased in these same groups. This suggests that venlafaxine exposure altered placental development even at doses that did not cause maternal toxicity or fetal growth restriction.

External abnormalities. One live fetus with a "dolphin-shaped" appearance was found in the 100 mg/kg/day group. No other major external anomalies were observed, except for an increased incidence of subcutaneous hematomas (bruises) commonly located between the shoulder blades in the 30 mg/kg/day group.

Drug levels confirmed. In the 30 mg/kg/day group, the plasma venlafaxine concentration was measured at 1.63 ± 0.47 μg/L, while the concentration of the major active metabolite, desvenlafaxine, was 7.06 ± 2.05 μg/L. This confirms that the drug was absorbed and metabolized in the rats, producing the same active metabolite that is found in humans taking venlafaxine.

Key Findings: Heart Defects in Fetal Rats

This was the core finding of the study. The researchers examined the hearts of 506 fetuses and identified several types of cardiac anomalies:

• Pulmonary trunk dilatation — widening of the main artery leading from the heart to the lungs
• Absence of the innominate artery — a missing blood vessel in the chest
• Enlargement of the left or right atrium — the upper chambers of the heart
• Enlargement of the right ventricle — the lower right chamber of the heart
• Ventricular septal defects (VSDs) — holes in the wall separating the two lower chambers of the heart

The total number of cardiac anomalies (all types combined) was significantly elevated in the venlafaxine-treated groups compared to control. When the researchers broke down the results by specific defect type and dose, a clear pattern emerged:

• Pulmonary trunk dilatation: Significantly increased in the 3 mg/kg/day group (5.00 ± 0.55 vs. 2.55 ± 0.41 in controls, p<0.001) and in the 10 mg/kg/day group (4.33 ± 0.40, p<0.05).
• Enlarged left atrium: Significantly increased in the 10 mg/kg/day group (4.42 ± 0.42 vs. 2.10 ± 0.51 in controls, p<0.05).
• Enlarged right ventricle: Significantly increased in the 10 mg/kg/day group (2.50 ± 0.58 vs. 0.85 ± 0.18, p<0.05) and even more dramatically in the 30 mg/kg/day group (3.75 ± 0.64, p<0.001).
• Ventricular septal defects: Significantly increased in the 10 mg/kg/day group (2.17 ± 0.46 vs. 0.65 ± 0.23 in controls, p<0.05).

The number of fetuses per litter with ventricular septal defects was also significantly increased in the 10 mg/kg/day treatment group compared to controls.

A striking sex difference emerged. When the researchers analyzed the data separately for males and females, they found that the incidences of both enlarged right ventricles and ventricular septal defects were significantly increased above control in male fetuses but not female fetuses. However, a direct statistical comparison between the sexes did not reveal a significant difference, meaning the sex difference should be interpreted cautiously.

Key Findings: Changes in Serotonin Signaling Genes

To understand the biological mechanism behind the heart defects, the researchers analyzed gene expression in the placenta and fetal hearts. They did not include the 100 mg/kg/day group in these analyses due to maternal toxicity.

In the placenta:

• NET/Slc6a2 mRNA levels were not affected by venlafaxine treatment. So the drug did not appear to affect norepinephrine transporter expression in the placenta.
• MAO-A/Maoa mRNA levels were decreased in the 30 mg/kg/day group (p<0.05). This is the enzyme that normally degrades (breaks down) serotonin, so reduced MAO-A could lead to higher serotonin levels.
• SERT/Slc6a4 mRNA levels were decreased in ALL venlafaxine-treated groups (3, 10, and 30 mg/kg/day), compared to control (p<0.05).
• SERT protein levels were significantly decreased in the 30 mg/kg/day group, confirming that the reduced gene expression translated into reduced actual protein.

In short, venlafaxine treatment decreased the expression of genes that regulate serotonin transport and degradation in the placenta, while having no effect on the norepinephrine transporter. This means the drug is specifically disrupting serotonin processing at the level of the placenta.

In the fetal heart:

• MAO-A/Maoa mRNA levels were not affected by venlafaxine treatment.
• Htr2b (serotonin 2B receptor) mRNA levels were significantly increased in the 3 and 10 mg/kg/day venlafaxine treatment groups. The serotonin 2B receptor is known to be critical for normal heart development.
• SERT/Slc6a4 expression showed a striking sex difference: In the hearts of male fetuses, SERT mRNA levels were not affected by venlafaxine. But in female fetal hearts, there was a 3-fold increase in SERT mRNA expression in the 10 mg/kg/day treatment group.
• SERT protein levels were increased after venlafaxine exposure (at 3 and 10 mg/kg/day) but only in female hearts.

Effects on the FGF8/NODAL/PITX2 pathway: Since serotonin guides early heart development by stimulating this critical left/right signaling pathway, the researchers examined whether venlafaxine disrupted it. They found that Fgf8 mRNA levels were increased in the fetal heart with exposure to venlafaxine. However, the mRNA levels of Nodal and Pitx2 — the downstream components of this pathway — were unchanged.

This suggests that venlafaxine exposure does alter the FGF8 component of this important developmental pathway, even though downstream signaling appears to be maintained. The precise implications of this disruption require further study.

What This Means for Patients

This study provides important evidence that venlafaxine, at doses equivalent to those used in humans, can disrupt fetal heart development in an animal model. The researchers were careful to note that the 3 and 10 mg/kg/day doses used in this rat study correspond to the therapeutic doses of venlafaxine used in humans (75 to 150 mg/day), especially when adjusted for the well-established difference in body weight-to-surface-area ratio between rats and humans (a factor of 6.9).

The finding that desvenlafaxine — the major active metabolite of venlafaxine found in humans — was detected in the blood of the treated rats is important because it confirms that the rat model metabolizes the drug similarly to humans.

The proposed mechanism is that venlafaxine disrupts the normal function of the serotonin system in both the placenta and the developing fetal heart. Specifically:

• In the placenta, venlafaxine reduces the expression of SERT (the serotonin transporter) and MAO-A (the serotonin-degrading enzyme), which would alter the amount of serotonin reaching the fetus.
• In the fetal heart, venlafaxine increases the expression of the serotonin 2B receptor and, in females, increases SERT expression. These changes could directly interfere with the signaling pathways that guide normal heart formation.

The study's findings are particularly relevant because ventricular septal defects are the most common type of congenital heart defect in humans, and they account for a significant portion of infant mortality. The lead researchers propose that "the increased incidence of cardiac anomalies is mediated through alterations in serotonin signaling in the placenta and fetal heart."

It is critical to emphasize that this is a rat study, and the results cannot be directly extrapolated to humans. However, studies like this are essential because they can reveal biological mechanisms that may also be at play in humans, and they can guide the design of better human studies.

Study Limitations

Readers should understand the limitations of this research before drawing conclusions:

• Animal model: This study was conducted in rats, not humans. While rats are a valuable model for developmental biology, there are important species differences in drug metabolism, placental structure, and fetal development that may affect how these findings translate to humans.
• Healthy animals: The rats in this study were healthy and did not have depression. In humans, depression itself can affect pregnancy outcomes, and the interaction between depression and antidepressant exposure cannot be captured in this model.
• Dosing differences: The drug was given by gavage (forced feeding) once daily, resulting in high peak blood levels. In humans, venlafaxine is typically given as an extended-release formulation that produces more stable blood levels throughout the day. The pattern of exposure may be different.
• No long-term follow-up: The fetuses were examined at gestation day 21 (the day before birth). The study does not tell us whether these heart anomalies would persist after birth, cause functional problems, or resolve on their own.
• Conflicting background literature: The authors acknowledge that previous animal studies with venlafaxine did not find increased malformations, and many human epidemiological studies have not found statistically significant increases in heart defects with SSRI/SNRI use. The scientific picture is not yet clear.
• Sex differences uncertain: Although some findings appeared sex-specific (such as increased SERT expression in female fetal hearts), the overall cardiac anomaly rates did not differ significantly between males and females, so the meaning of the sex differences remains unclear.

Recommendations for Patients and Healthcare Providers

This study should not be interpreted as a reason to stop antidepressant medication abruptly. The researchers themselves emphasize that untreated depression during pregnancy carries its own significant risks.

If you are pregnant, planning to become pregnant, or currently taking venlafaxine (Effexor XR) or another antidepressant, here is what the authors of this study and general medical guidance suggest:

1. Do not stop your medication without talking to your doctor. Abruptly stopping an antidepressant can cause withdrawal symptoms and a return of depression symptoms, which carry their own risks for both mother and baby.
2. Have an open conversation with your healthcare provider. Discuss the potential risks and benefits of continuing, changing, or stopping your antidepressant during pregnancy. Your doctor can help you weigh the risks of untreated depression against the potential risks of medication exposure.
3. Ask about fetal monitoring. If you are taking venlafaxine during pregnancy, your doctor may recommend fetal echocardiography (a specialized ultrasound of the baby's heart) around 18–22 weeks of gestation to assess the structure of the fetal heart.
4. Consider all treatment options. For some women, psychotherapy (such as cognitive behavioral therapy) may be an effective alternative or addition to medication. However, for women with moderate-to-severe depression, medication may be necessary and appropriate.
5. Know that the scientific picture is still evolving. This study adds important evidence about the potential risks of venlafaxine, but it is one study in rats. Researchers continue to investigate the safest approaches to treating depression during pregnancy.

The bottom line: The decision to take any medication during pregnancy is a deeply personal one that must be made in partnership with a healthcare provider who knows your individual medical history, mental health needs, and risk factors. The researchers in this study did not call for a halt in prescribing venlafaxine — rather, they emphasized the need for better information to help patients and doctors make informed decisions.

Frequently Asked Questions

I'm pregnant and taking venlafaxine. Should I stop my medication?

Do not stop your medication without talking to your doctor. Abruptly stopping an antidepressant can cause withdrawal symptoms and a return of depression symptoms, which carry their own risks for both mother and baby. The researchers emphasize that untreated depression during pregnancy carries significant risks. Discuss the potential risks and benefits of continuing, changing, or stopping your antidepressant with your healthcare provider.

What did this research find about venlafaxine and heart defects?

In a rat study, venlafaxine exposure during pregnancy increased the total number of fetal cardiac anomalies, including pulmonary trunk dilatation, enlarged left atrium, enlarged right ventricle, and ventricular septal defects. These effects appeared at doses equivalent to therapeutic human doses. The researchers propose the increased cardiac anomalies are mediated through alterations in serotonin signaling in the placenta and fetal heart.

Can these rat study results be directly applied to my pregnancy?

No. This was a rat study, and the results cannot be directly extrapolated to humans. There are important species differences in drug metabolism, placental structure, and fetal development. However, the researchers note that the 3 and 10 mg/kg/day doses correspond to therapeutic human doses of 75 to 150 mg/day, and the rat model produced desvenlafaxine, the same active metabolite found in humans.

What is a ventricular septal defect and why is it important?

A ventricular septal defect (VSD) is a hole in the wall separating the heart's lower chambers. It is the most common type of congenital heart defect in humans and accounts for a significant portion of infant mortality. In this rat study, VSDs were significantly increased in the 10 mg/kg/day group and in male fetuses, though a direct statistical comparison between sexes was not significant.

If I take venlafaxine during pregnancy, what monitoring might my doctor recommend?

Your doctor may recommend fetal echocardiography, a specialized ultrasound of the baby's heart, around 18–22 weeks of gestation to assess the structure of the fetal heart. Have an open conversation with your healthcare provider about the potential risks and benefits of continuing, changing, or stopping your antidepressant during pregnancy, weighing untreated depression risks against medication exposure risks.

What are the limitations of this study?

This was an animal study in healthy rats without depression, so it cannot capture how depression itself affects pregnancy. The drug was given by gavage once daily, producing high peak blood levels, unlike extended-release human dosing. Fetuses were examined the day before birth, so it is unknown whether heart anomalies persist after birth. Previous animal studies did not find increased malformations, and human epidemiological results are conflicting.

What treatment options exist for depression during pregnancy besides medication?

For some women, psychotherapy such as cognitive behavioral therapy may be an effective alternative or addition to medication. However, for women with moderate-to-severe depression, medication may be necessary and appropriate. The decision to take any medication during pregnancy is deeply personal and must be made in partnership with a healthcare provider who knows your individual medical history, mental health needs, and risk factors.

I'm pregnant and taking venlafaxine (Effexor XR) — when should I seek a second opinion about whether to continue it?

Seek a second opinion when you and your prescriber disagree about continuing, changing, or stopping venlafaxine during pregnancy, or when you want the risks of untreated depression weighed against medication exposure by someone who has not already advised you. A second opinion can also review whether fetal echocardiography around 18–22 weeks is appropriate for you. Do not stop the medication on your own; abrupt discontinuation can cause withdrawal and return of depression symptoms. Diagnostic Detectives Network provides independent expert second opinions.

Source Information

Original article title: In Utero Exposure to Venlafaxine, a Serotonin–Norepinephrine

Authors: Laetitia Laurent, Chunwei Huang, Sheila R. Ernest, Anick Berard, Cathy Vaillancourt, and Barbara F. Hales

Journal: Birth Defects Research (Part A), 2016. Published online by Wiley Periodicals, Inc. DOI: 10.1002/bdra.23537

Author affiliations: INRS-Institut Armand-Frappier (Laval, QC, Canada), McGill University Department of Pharmacology and Therapeutics (Montreal, QC, Canada), University of Montreal Faculty of Pharmacy and Research Center CHU Sainte-Justine (Montreal, QC, Canada)

Funding: This research was supported by the Réseau Québécois de Recherche sur les Médicaments, the Research Chair on Medications and Pregnancy, the Fonds de Recherche du Québec Santé, McGill University, March of Dimes Social and Behavioral Sciences Research, and the Réseau Québécois de la Reproduction.

Note: This patient-friendly article is based on peer-reviewed research. It is intended for educational purposes and does not constitute medical advice. Always consult a qualified healthcare provider regarding medical decisions.