Smoked Cigarettes Before You Knew You Were Pregnant?

What does the evidence say about smoked cigarettes before you knew you were pregnant during pregnancy?

1. Even a few cigarettes a day during pregnancy can lower the oxygen and nutrients reaching the fetus, and the more cigarette smoke exposure during pregnancy, the greater the chance of preterm delivery. 1
2. Cigarette smoke contains more than 4,000 chemicals, including nicotine, arsenic, lead, and carbon monoxide, and exposure during pregnancy has been linked to miscarriage, certain birth defects, preterm delivery, low birth weight, and stillbirth. 2
3. In a study following pregnancies, people who stopped smoking in early pregnancy had no increase in the risk of a small-for-gestational-age birth or childhood overweight at age three, with birth weights comparable to people who never smoked. 3
4. Quitting smoking at any stage of pregnancy is associated with improved outcomes, and stopping before 15 weeks specifically has been shown to lower the chance of preterm birth and a small-for-gestational-age baby to about the same level as someone who never smoked. 4
5. In one longitudinal cohort, 55.7% of pregnant smokers had already made a quit attempt by the time they were surveyed in early pregnancy, and researchers note that most people who successfully quit do so soon after finding out they're pregnant, often within days. 5

Is smoked cigarettes before you knew you were pregnant safe in each trimester?

• First trimester. This worry almost always centers on the first trimester, since that's the stretch between conception and a positive test. Research on people who stop smoking in early pregnancy has found growth outcomes similar to those who never smoked, and providers routinely support people quitting at this stage.
• Second trimester. If you smoked briefly before finding out and then quit, there's no research suggesting that early, pre-recognition exposure carries added risk once you're into the second trimester. The relevant question becomes whether smoking has continued, not what happened before you knew.
• Third trimester. By the third trimester, the guidance is the same as it's been throughout: continued smoking still carries the same risks, including preterm delivery and low birth weight, regardless of what happened before you found out. If you already quit, that choice continues to matter more than the early exposure did.