Health experts do not know of a set amount of drinking that causes fetal alcohol syndrome (FAS). Several factors influence the risk, including how much a person drinks and their overall health status, environment, and genetics.

This information comes from the National Institutes of Health (NIH).

Because alcohol can affect fetal development at any stage of pregnancy, health authorities state that no amount of alcohol consumption is safe during pregnancy. There is also no safer type of alcoholic drink that is less likely to cause harm than others.

FAS can cause a wide variety of lifelong symptoms and complications, including changes in growth, facial features, learning, mood, and behavior. Stopping drinking as early in pregnancy as possible is the best way to prevent the condition.

Read on to learn more about how much drinking causes FAS.

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FAS belongs to a spectrum of conditions that occur when a fetus has exposure to alcohol during pregnancy. Doctors call these conditions fetal alcohol spectrum disorders (FASDs).

FAS is the most severe form of FASD. Other types include:

  • alcohol-related neurodevelopmental disorder, which involves intellectual and learning disabilities
  • alcohol-related physical differences in growth and development
  • neurobehavioral disorder associated with prenatal alcohol exposure, which involves changes in behavior and mood

Children with FAS often have a combination of these effects. FAS affects many aspects of development, including:

  • memory
  • speech
  • coordination
  • attention
  • vision
  • hearing

It can also affect the growth of a fetus, resulting in:

  • small facial features
  • a small head
  • below-average height
  • low birth weight
  • problems with the lungs, heart, or bones

Learn more about FAS symptoms and treatment.

No specific amount of alcohol will cause FASDs in every person. Alcohol may affect fetal development differently from person to person and from pregnancy to pregnancy.

Even if a person drank some alcohol during a previous pregnancy and had a healthy baby, it does not mean future pregnancies will be the same. This is because many factors influence the risk, including:

  • how much a person drinks
  • how frequently they drink
  • at what stage of fetal development they drink — this can affect what symptoms a baby with FAS might have
  • their genetics

Environmental factors can also raise the risk. Alcohol consumption is more likely to affect a fetus if the pregnant person:

  • smokes
  • is older
  • has a lower-than-average height or weight
  • does not get proper nutrition
  • does not have proper access to prenatal care
  • experiences high levels of stress
  • has had multiple pregnancies
  • comes from a family of people who drink heavily
  • lives in an environment in which alcohol misuse is common

However, even if none of these risk factors apply to a person, their baby may still develop FASD. No amount of alcohol consumption is safe during pregnancy.

In general, the less a person drinks, the less likely FAS is. However, any amount of alcohol can be harmful, even just one drink.

A 2020 study found that children whose birth parent drank any amount of alcohol during pregnancy were more likely to have:

  • difficulty paying attention
  • impulsiveness
  • a mental health diagnosis such as separation anxiety or oppositional defiant disorder

Light alcohol use had a connection to a lower risk of these outcomes, but an association was still present.

Scientists are still learning about the ways alcohol affects fetal development. Having one drink during pregnancy may not lead to FAS, but because a fetus cannot process alcohol effectively, any amount could cause harm. The only way to be certain that alcohol will not affect a pregnancy is to avoid consuming alcohol.

Many people drink, either intentionally or unintentionally, during pregnancy. According to a 2009 study, 20–30% of women report drinking at some point during a pregnancy, often during the first trimester.

This can happen if a person becomes pregnant unexpectedly. Weeks may pass before they realize they are pregnant, and they may drink alcohol during this time without knowing the risk.

If this happens, it does not necessarily mean that their child will have FASD. The amount and frequency of the drinking matters, as well as how quickly a person stops.

If a person has consumed alcohol during pregnancy, they should speak with a doctor about their concerns. This applies no matter what stage of the pregnancy they are in or how much alcohol they have consumed.

It is never too late to stop drinking during pregnancy. The earlier a person can stop, the better it is for them and for the fetus.

A doctor may be able to provide support if a person is having difficulty stopping. Alternatively, support organizations can provide confidential support and advice.

Seeking help for addiction may seem daunting or even scary, but several organizations can provide support. If you believe that you or someone close to you is showing signs of addiction, you can contact the following organizations for immediate help and advice:

Below are some answers to frequently asked questions about alcohol and pregnancy.

How common is FAS?

It is difficult to estimate how common FAS and other FASDs are, as the symptoms can be similar to those of other conditions. Additionally, because of stigma, some people may not disclose drinking during pregnancy.

In a 2017 analysis, researchers looked at data from 24 previous studies from around the world. They estimated that FASD affects 7.7 out of every 1,000 people worldwide. The rates vary significantly by location.

In some countries, the rates of FASD are significantly higher than average. The condition affects:

  • 111.1 out of every 1,000 people in South Africa
  • 53.3 out of every 1,000 people in Croatia
  • 47.5 out of every 1,000 people in Ireland

In which trimester does FAS develop?

FAS does not develop in one specific trimester. Alcohol can affect fetal growth and the development of the brain and nervous system at any time during pregnancy.

Changes in the development of facial features tend to happen in the first 3 months, when the fetus is still very small.

Does the placenta filter alcohol?

No, the placenta does not filter alcohol from a person’s blood. Alcohol transmits easily from a pregnant person’s bloodstream to the bloodstream of the fetus. Because the fetus’s body cannot process alcohol well, alcohol can stay in the body for a long time.

Health authorities do not know of any safe amount of alcohol that a person can drink during pregnancy. Additionally, no set amount of alcohol causes FAS or other FASDs in every case. The volume of alcohol a person consumes has a significant effect on the risk, but other factors can also play a role.

Genetics, environment, and a pregnant person’s overall health and body weight may determine the likelihood of FAS. However, even if a person has low risk, light drinking can affect fetal brain development. Scientists are still learning about this.

FAS is preventable. If a person is concerned about drinking during pregnancy or worried that their baby may have FAS, they should speak with a doctor as soon as they can.