Losartan During Pregnancy: Risks and What to Know

What does the evidence say about losartan during pregnancy?

1. Losartan hasn't been well studied in the first trimester, and it isn't known whether it raises the chance of birth defects at that stage; one small study of ARB-exposed pregnancies reported two birth defects among 20 pregnancies, though a cause-and-effect link wasn't established. 1
2. Using losartan in the second and third trimesters can cause low levels of amniotic fluid, which can lead to poor lung and skull development, joint contractures, and growth restriction; low blood pressure and kidney disease or failure have also been reported in exposed babies, and some cases have resulted in fetal death. 2
3. The renin-angiotensin system plays an essential role in fetal kidney development that continues throughout pregnancy, so blocking it with losartan reduces fetal urine production; the resulting low amniotic fluid can compress the fetus and lead to underdeveloped lungs, which is why documented harm is worse with exposure later in pregnancy rather than earlier. 3
4. Because of these risks, clinical guidelines recommend stopping ACE inhibitors or ARBs like losartan as soon as pregnancy is confirmed, ideally within about 2 working days of notification, and starting an alternative; labetalol is generally the preferred first-line blood pressure medication in pregnancy, with nifedipine or methyldopa as alternatives. 4
5. MotherToBaby's guidance for someone who is pregnant and taking losartan is to talk with a healthcare provider before stopping the medication, since some untreated health conditions can also raise pregnancy risks, so the switch to another medication should be managed rather than done abruptly alone. 1

Is losartan safe in each trimester?

• First trimester. Losartan hasn't been well studied for first-trimester use, and it isn't known whether it raises the chance of birth defects at this stage. Because of that uncertainty and the more serious risks later in pregnancy, guidelines recommend switching away from it as soon as pregnancy is confirmed.
• Second trimester. Second-trimester exposure is where the documented risks become clear: losartan can reduce amniotic fluid and affect how fetal kidneys develop, since the renin-angiotensin system it blocks plays an ongoing role in fetal kidney maturation through this stage.
• Third trimester. Third-trimester exposure carries the most severe documented risks, including low amniotic fluid, poor lung development from compression, and kidney problems in the newborn. If you're still taking losartan at this point, this is an urgent conversation to have with your provider about switching medications.