How gestational age is measured
Gestational age counts from the first day of the last menstrual period (LMP), not from conception. This convention means pregnancy is technically "counted" from about 2 weeks before fertilisation actually occurs. A full-term pregnancy is 40 weeks (280 days) from LMP.
If your cycle is longer or shorter than 28 days, ovulation (and likely conception) shifts: each day your cycle differs from 28 days shifts the ovulation date by one day. Our calculator adjusts the LMP-based estimate for cycle length.
Trimester breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1–12 | Organ formation, heartbeat (wk 6), NT scan (wk 11–13) |
| Second | 13–26 | Anatomy scan (wk 18–20), viability threshold (~22–24 wk) |
| Third | 27–40 | Lung maturation, brain development, GBS test (wk 36) |
Sources
- ACOG Committee Opinion No. 700. "Methods for Estimating the Due Date." Obstetrics & Gynecology. 2017;129(5):e150–e154.
- Naegele FC. Erfahrungen und Abhandlungen aus dem Gebiete der Krankheiten des weiblichen Geschlechtes. Mannheim, 1812.