If you are pregnant or trying to become pregnant, you have probably heard that it is best not to drink any alcohol. That’s the recommendation of the United States Centers for Disease Control and Prevention, as well as the American College of Obstetricians and Gynecologists. [1,2] There are some great options for mocktails out there, yet because pregnancies are often surprises, some women drink alcohol before they even know they are pregnant. And other pregnant people might like to have a drink every now and then. Read on for a discussion of everything you need to know about drinking alcohol during pregnancy.
Before You’re Pregnant
The main reason that official recommendations advise women not to drink if they are trying to conceive is that there is evidence that even light alcohol use—that is, one to two drinks per week—can affect fertility and increase the risk of miscarriage. For instance, one study showed that consuming alcohol could make in vitro fertilization less successful.  Other studies in mice [4,5] and monkeys  have shown that alcohol consumption can lead to chromosomal changes in eggs that increase the chance of miscarriage. The tricky thing about all these studies is that most people metabolize alcohol differently, so an amount of drinking that could impact one person’s fertility might not have the same effects on someone else’s experience of trying to get pregnant. 
But what if you have already had some drinks and find out you’re pregnant? The good news is that early embryos are pretty resilient and that alcohol probably doesn’t cross the placenta until the third week after conception, so week five or so of pregnancy.  If you have had a few drinks, you can always check in with your care provider, but your baby is probably okay.
After week five or so of pregnancy, alcohol can cross the placenta, which means that it can enter your baby’s developing body through the umbilical cord. This is another place where animal studies and human evidence have shown that alcohol can be damaging to developing babies.
Fetal Alcohol Spectrum Disorders, which are a group of conditions that can happen to your baby when a pregnant person drinks alcohol, are what most people worry about when they advise against drinking alcohol during pregnancy. Researchers know that heavy alcohol use, which might look like binge drinking—drinking three or more drinks in one occasion—or like having six or more drinks per week for more than two weeks during pregnancy, can lead to Fetal Alcohol Spectrum Disorders. 
As the name implies, there is a wide spectrum of symptoms that arise from fetal exposure to alcohol.  Some possible physical symptoms include:
- Small head size
- Short stature and low birth and body weight
- Unusual facial features
- Problems with the heart, kidneys, or bones
Fetal Alcohol Spectrum Disorders can also result in cognitive issues that may last through childhood, into adulthood, and beyond and can include:
- Hyperactivity and attention issues, which can lead to difficulty in school
- Learning disabilities, which can also lead to difficulty in school
- Low IQ
If binge drinking or moderate to heavy drinking is something you are struggling with during pregnancy, speak to your healthcare provider. There are lots of resources—many of them low cost or free—that can help you stop drinking while you are pregnant. 
What about light drinking?
While it’s clear that heavy drinking is not best for baby, many pregnant women wonder if the occasional drink—especially if it happens during the second or third trimesters, when much of critical fetal development has already occurred—is okay.
Behavioral economist Emily Oster, who evaluates the research on light drinking in her book Expecting Better, maintains that it is probably fine to have a drink per week during the first trimester and up to one drink per day during your second and third trimesters.  In an essay for Expecting Science, researcher Amy Kiefer discusses some of the science around light drinking in pregnancy and brings up the important point that alcohol affects different people—and thus likely their unborn babies—differently. 
Ultimately, the decision to have an occasional glass of wine with dinner or to participate in a champagne toast is up to you. Your care provider might have good insight on this question, and talking to him or her could help inform your decisions.
1. CDC, Alcohol Use in Pregnancy
2. ACOG, Alcohol and Women
3. P. Nicolau et al., “Alcohol consumption and in vitro fertilization: a review of the literature,” Gynecological Endocrinology, 2014.
4. E. Cebral et al., “Impaired Mouse Fertilization by Low Chronic Alcohol Treatment,” Alcohol and Alcoholism, 1997.
5. M.H. Kaufman, “The teratogenic effects of alcohol following exposure during pregnancy, and its influence on the chromosome constitution of the pre-ovulatory egg,”Alcohol and Alcoholism, 1997.
6. M.L. Schneider et al., “Moderate Alcohol Consumption and Psychological Stress during Pregnancy Induce Attention and Neuromotor Impairments in Primate Infants,” Child Development, 1997.
7. Expecting Science, Light Drinking During Pregnancy
8. L. Denney et al., “Fetal Alcohol Syndrome and Fetal Alcohol Spectrum Disorders,” American Family Physician, 2017.
9. Emily Oster, Slate, “I Wrote That It’s OK to Drink While Pregnant. Everyone Freaked Out. Here’s Why I’m Right.”