Pregnancy Week Calculator

Pregnancy Week Calculator Guide

Understand the 3 calculation modes, the formulas behind them, weekly fetal milestones, and the recommended visit schedule.

1. Three calculation modes

A. LMP (last menstrual period) — the most common starting point for regular cycles. Formula: EDD = LMP + 280 days (Naegele rule).

B. Ovulation — preferred when cycles are irregular or ovulation was tracked (LH kit, BBT). Formula: EDD = Ovulation + 266 days.

C. Ultrasound — the most accurate method in the first trimester, based on CRL. Enter scan date and gestational age at scan; we back-calculate an effective LMP and due date.

2. Understanding the Naegele rule

Gestational age is measured from the first day of the last menstrual period, not conception. Since average ovulation occurs around LMP + 14 days, fetal age is roughly GA − 2 weeks. 40 weeks is convention; 37–42 weeks is the accepted full-term window.

3. Ultrasound accuracy and re-dating

  • First trimester (6–13w) CRL: ±5–7 days, most accurate
  • Second trimester (14–20w) BPD/FL: ±7–10 days
  • Third trimester: ±2–3 weeks; re-dating is rarely useful
  • When LMP and ultrasound disagree by > 7 days, obstetricians usually adopt the ultrasound date.

4. Weekly fetal development at a glance

  • First trimester (1–13w): conception, implantation, organogenesis. Highest miscarriage risk.
  • Second trimester (14–27w): movement, anatomy scan, rapid skeletal and muscular growth. Most stable period.
  • Third trimester (28–40w): weight gain, lung maturation, head-down positioning. Viability at 24w, full term from 37w.

5. Recommended prenatal visit schedule

  • 8w · first visit (ultrasound + labs)
  • 12w · NT (nuchal translucency) scan
  • 16w · quad screen
  • 20w · anatomy ultrasound
  • 24w · routine check-up
  • 28w · glucose challenge test (GDM)
  • 32·34·36w · transition to weekly visits, GBS swab at 36w
  • 37–40w · weekly, cervix checks

Download the ICS file from the calculator to auto-import these visits into Google or Apple Calendar.

6. Medical disclaimer

All calculations are estimates only. Always consult your OB/GYN for accurate gestational age and due date. Seek medical care immediately for heavy bleeding, severe abdominal pain, or significantly reduced fetal movement.

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