Gestational Age Calculator
1LMP or EDD Dating
2Ultrasound Dating
Breakdown
What Is Gestational Age?
Gestational age (GA) is how far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period (LMP) — not from conception. Because conception typically happens about two weeks after the LMP, gestational age runs roughly two weeks ahead of fetal age: a pregnancy described as “10 weeks” is about 8 weeks post-conception. Obstetrics dates from the LMP anyway, because a period is an observable event while the exact day of conception almost never is. The convention writes gestational age as weeks plus days, so “10w 3d” means 10 weeks and 3 days.
How OB Dating Works
LMP dating: Naegele’s rule
A full-term pregnancy is counted as 40 weeks (280 days) from the LMP, so the estimated due date (EDD) is simply the LMP plus 280 days — Naegele’s rule, which assumes a regular 28-day cycle with ovulation around day 14:
Estimated due date from the last menstrual period:
EDD = LMP + 280 days (40 weeks)Ultrasound dating: measuring the embryo
An early ultrasound measures the embryo directly — in the first trimester, the crown-rump length (CRL). Early embryos grow at a remarkably uniform rate, so the CRL maps tightly onto gestational age. The scan reports an estimated gestational age (EGA) for that day, and the due date follows by adding whatever is left of the 280 days:
Estimated due date from an ultrasound:
EDD = Scan Date + (280 − EGA at scan, in days)Early ultrasound is the most accurate dating method because embryos of the same age are nearly the same size before about 14 weeks. Later on, healthy babies diverge in size, so a scan’s dating margin widens from a few days in the first trimester to two or three weeks near term.
Worked Example
LMP dating. Suppose the LMP was July 30, 2026. Adding 280 days gives an EDD of May 6, 2027. Seventy days later — October 8, 2026 — the pregnancy is at exactly 10w 0d.
Ultrasound dating. Now a different pregnancy: a scan on July 30, 2026 measures 10 weeks 5 days (75 days). The remaining span is 280 − 75 = 205 days, so the sono-based EDD is February 20, 2027.
Comparing the two. When both are available for the same pregnancy, they rarely agree exactly. Say the LMP was June 14, 2026 (LMP EDD March 21, 2027), and a scan on August 16, 2026 measures 9w 3d (sono EDD March 18, 2027). The due dates differ by 3 days — within the 7-day threshold for a 9-week scan, so the LMP date would typically be kept.
When Ultrasound Overrides LMP
ACOG (the American College of Obstetricians and Gynecologists) publishes thresholds for when the ultrasound date should replace the LMP date. The earlier the scan, the more it is trusted. If the two EDDs differ by more than the threshold for the scan’s timing, the pregnancy is redated to the ultrasound:
| EGA at ultrasound | Redate if EDDs differ by |
|---|---|
| Up to 8w 6d | more than 5 days |
| 9w 0d – 15w 6d | more than 7 days |
| 16w 0d – 21w 6d | more than 10 days |
| 22w 0d – 27w 6d | more than 14 days |
| 28w 0d and later | more than 21 days |
Once a due date is established this way, it is not supposed to change again — later scans that measure “ahead” or “behind” reflect growth, not a new due date. The calculator’s guidance row applies these thresholds to your entries so you can see how the decision would be made, but the actual call always belongs to your provider.
Common Mistakes to Avoid
The most common mix-up is confusing gestational age with fetal age. Gestational age starts about two weeks before conception, so “8 weeks pregnant” does not mean an 8-week-old embryo; comparing the two conventions creates a phantom two-week discrepancy.
Irregular cycles quietly break LMP dating. Naegele’s rule bakes in ovulation on day 14 of a 28-day cycle. With long cycles, late ovulation, recent hormonal contraception, or breastfeeding, the LMP date can be off by a week or more — exactly the situation where an early ultrasound matters most.
Using late ultrasounds to redate is the third trap. A third-trimester scan that measures “two weeks ahead” is far more likely to describe a larger-than-average baby than a wrong due date — which is why the threshold at 28 weeks is a full 21 days, and why an established due date should not be moved by a late growth scan.
Frequently Asked Questions
Why is pregnancy 40 weeks (280 days)?
The 280-day figure is 10 lunar months, counted from the LMP rather than from conception because the LMP is observable. Since conception happens about two weeks in, the embryo actually develops for roughly 38 weeks (266 days). The 280-day EDD is a statistical midpoint, not a deadline — only about 1 in 20 babies arrives on the due date itself, and delivery anywhere from 37 to 42 weeks is considered within the normal range.
What if I don’t know my LMP?
Then dating relies on ultrasound, which is the standard of care in that situation. Enter only the ultrasound panel in this calculator and it will compute the EGA and EDD from the scan alone. A first-trimester scan dates a pregnancy to within about 5–7 days even with no LMP at all. Uncertain LMP, irregular cycles, and conception while on contraception are all routine reasons pregnancies are dated by scan.
Which date wins if my LMP and ultrasound disagree?
It depends on how big the gap is and how early the scan was. ACOG’s rule of thumb: a scan up to 8w 6d overrides the LMP if the due dates differ by more than 5 days; at 9–15 weeks the margin is 7 days; and it widens to 10, 14, and 21 days for later scans. Inside the margin, the LMP date is kept. Your provider makes this call once, early, and the established due date then stays fixed.
Is this calculator a substitute for my provider’s dating?
No. It is an educational model of how the arithmetic works. Real OB dating folds in cycle history, embryo measurements from the actual scan images, IVF transfer dates when relevant, and clinical judgment. Use this page to understand where the numbers come from — and use your healthcare provider’s dating for every real decision about a pregnancy.