Pregnancy Weeks: Understanding Gestational Age and Conception

Pregnancy dating uses gestational age (GA), not conceptional age.

Key Definitions

• LMP (Last Menstrual Period): Day 1 of last menses = GA 0 weeks 0 days.
• Conception: Typically ~14 days after LMP (ovulation + fertilization).
• Embryonic Age: GA minus 2 weeks (post-conception).

Why This Matters

• At "4 weeks pregnant" (GA), embryo is ~2 weeks post-conception.
• Ultrasound measures embryonic/fetal size → estimates GA (assumes LMP dating).
• Due date (EDD) = LMP + 280 days (40 weeks). Naegele's rule: LMP - 3 months + 7 days + 1 year.

Clinical Trimesters

• First: <14 weeks
• Second: 14–27 6/7 weeks
• Third: ≥28 weeks

Term Definitions

• Early term: 37 0/7 – 38 6/7 weeks
• Full term: 39 0/7 – 40 6/7 weeks
• Late term: 41 0/7 – 41 6/7 weeks
• Postterm: ≥42 0/7 weeks

Irregular Cycles / Unknown LMP

Early ultrasound (crown-rump length 7–13 weeks) is most accurate for dating (±5–7 days). Later ultrasounds less precise for dating.

GA standardization enables consistent communication, screening timing, and outcome comparison.