Why timing matters most in the first trimester

What does the evidence say about why the first trimester is the highest-risk window for exposures during pregnancy?

1. MotherToBaby explains that most major structural differences arise in the first trimester because the heart, limbs, lips, palate, and other structures form then, while functional effects can arise later. 1
2. Clinical pregnancy dating begins at the last menstrual period, about two weeks before conception, so embryology weeks and charted gestational weeks are not interchangeable. 2
3. Organogenesis occurs during embryonic weeks 3 through 8, when the three germ layers form developing organs; by week 9 the fetal period emphasizes growth and further differentiation. 3
4. A teratology review places organ formation from about 14 to 60 days after conception and emphasizes that susceptibility depends on exposure timing, dose, route, and maternal and fetal genotype. 4
5. CDC notes that fetal effects of a medicine or vaccine depend on the agent, dose, route, gestational timing, other exposures, the underlying infection, and maternal and fetal susceptibility. 5

Is why the first trimester is the highest-risk window for exposures safe in each trimester?

• First trimester. The first trimester contains the all-or-none interval and the embryonic organ-forming period. Most major structures form during this span, but each structure has its own narrower window of greatest sensitivity.
• Second trimester. Second-trimester exposure risk does not fall to zero. Structural effects may be less likely after an organ has formed, while growth, hearing, brain function, amniotic fluid, and organ function can remain vulnerable.
• Third trimester. Third-trimester exposures can still affect fetal growth, organ function, delivery timing, newborn adaptation, or withdrawal. A later exposure should be assessed by the specific agent rather than dismissed because organogenesis has passed.