Which of the following correctly matches trimester timing to a key concept?

First trimester = weeks 1–20 (critical period ends at week 20); Second = weeks 21–30; Third = weeks 31–40

First trimester = weeks 0–8; Second = weeks 9–20; Third = weeks 21–40

Trimesters are defined by equal 13-week divisions starting at conception, and there is no specific critical period for teratogens

First trimester = weeks 0–13 (critical period for teratogens); Second = weeks 14–27; Third = weeks 28–birth (~40 weeks)