June 26, 2022
There is a lack of safety data on medications during pregnancy. Since the exact mechanisms of action for many medications are not fully understood, drugs are best generally avoided during pregnancy when possible. There are, however, some conditions that demand the use of prescription or over-the-counter (OTC) medications. Most women use at least one type of medication during the course of pregnancy, with analgesics like acetaminophen (Tylenol) being one of the most common. Acetaminophen is generally considered safe at the recommended doses. Let’s dive a little deeper because it’s fun.
Maternal and fetal blood is separated throughout pregnancy. However, essential communication between the woman and the fetus facilitates pregnancy maintenance, nutritional exchange and removal of fetal waste products.
The placenta can allow medications, like acetaminophen, to pass freely to the fetus, completely block them from entering the fetus or break down the medication in to inert or toxic metabolites.
When we swallow an acetaminophen pill much of it stays a complete molecule in out blood and some of it is broken into metabolites by our liver.
Studies using the human placenta done in a lab (called ex-vivo studies) suggest acetaminophen freely passes through the placenta. Acetaminophen is a fairly small molecule (151 Da) and is lipophilic so can diffuse through the placenta. Acetaminophen metabolites are transported across the placenta imperfectly so they do not seem to be able to enter the fetus in high concentrations like the full acetaminophen molecule does.
So, if you take acetaminophen tablet in pregnancy the fetus will as well.
The most important question is: does acetaminophen use in pregnancy cause problems for the pregnancy or newborn?
The short answer seems to be “no.”
The longer answer is that we don’t know as well as we’d like. Pregnancy studies are hard to do - women to not want to be studied in pregnancy. So, studies are done on women who have a pregnancy problem or health issue in their newborn. For example, women who give birth to a child with a heart defect are then asked what they took in pregnancy - they are more likely to remember what they took than women who do not have an issue. Often women who take acetaminophen in pregnancy also take other medications. Some medications that contain acetaminophen also contain other medications. Some women take acetaminophen many days, and other take it just once through pregnancy. So there is so much bias in reporting that the results are hard to trust completely. All that messy bias aside, what are the possible concerns with acetaminophen use in pregnancy?
There may be an association with the child developing ADHD. Some studies suggested higher rates of asthma in children, but others did not. There does not seem to be an association with renal or heart disease. There does not seem to be an association with miscarriage.
As someone who has ADHD (self diagnosed; ok also diagnosed by many people who know me!) I’m not concerned about these (possible, unlikely) risks so my advice is take acetaminophen if you need pain relief in pregnancy. Do not take other analgesics (e.g. ibuprofen). If you need something stronger than acetaminophen please talk to your doctor.
References available on request.
Dr. Beth Taylor
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