Gestational age is how the progress of a pregnancy is measured โ the anchor for every scan schedule, screening window, and the estimated due date itself. Unusually, it doesn't count from conception. By convention, gestational age starts from the first day of the last menstrual period (LMP), roughly two weeks before conception typically occurs. That quirk exists because the LMP is a date most people can identify, while the moment of conception almost never is.
Method 1: The LMP calculation
The arithmetic is a straightforward day count:
Worked example: LMP on 10 April 2026, today is 12 July 2026. That's 93 days. 93 รท 7 = 13 weeks with 2 days left over, so the gestational age is 13 weeks 2 days โ written clinically as 13 2/7 weeks. You can get the exact day count between any two dates from our age calculator by entering the LMP as the "birth" date and today as the target date.
The due date: Naegele's rule
The estimated due date (EDD) is the LMP plus 280 days โ 40 weeks. The classic shortcut, Naegele's rule, approximates this without counting: take the LMP, add one year, subtract three months, add seven days. LMP of 10 April 2026 โ 17 January 2027. Because month lengths vary, the rule can land a day or two away from a true 280-day count (which here gives 15 January) โ one more reason clinical dating leans on exact day counts and ultrasound.
Method 2: Ultrasound dating
In the first trimester, embryos grow at a remarkably uniform rate โ uniform enough that a single measurement of crown-rump length (CRL), the embryo's length from head to bottom, converts directly to gestational age. According to the American College of Obstetricians and Gynecologists (ACOG), first-trimester ultrasound (up to 13 6/7 weeks) is the most accurate method of establishing gestational age, with a margin of error of roughly 3โ8 days.
Later in pregnancy, dating shifts to a combination of measurements โ biparietal diameter (head width), head circumference, femur length, and abdominal circumference โ and the error margin widens to about 7โ10 days in the second trimester, and more in the third. This is why an early scan "locks in" the dates: later measurements reflect the baby's individual growth as much as its age.
When LMP and ultrasound disagree
Clinicians follow a redating rule: if a first-trimester ultrasound differs from the LMP-based date by more than 7 days, the ultrasound wins and the due date is changed. Within 7 days, the LMP date stands. The allowed discrepancy grows with each trimester, matching the scan's widening error bars.
Gestational age vs fetal age
| Term | Counts from | At "8 weeks pregnant" |
|---|---|---|
| Gestational age | First day of the LMP | 8 weeks |
| Fetal (conceptional) age | Conception | ~6 weeks |
The two are always about two weeks apart. Everything you hear in a clinical setting โ "20-week scan", "full term at 39โ40 weeks" โ uses gestational age. Full term at 40 weeks therefore means about 38 weeks of actual fetal development.
Why the number matters
Gestational age drives clinical timing: screening tests only valid within specific week windows, medication decisions, the classification of a birth as preterm (before 37 weeks) or post-term (after 42 weeks), and growth assessment against week-by-week norms. A dating error of even a week can shift a screening result from "normal" to "flagged", which is why so much care goes into establishing the number early.
Frequently asked questions
Need an exact day count between two dates?
Enter any start date and target date โ the calculator returns the precise gap in days and weeks.
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