By Zencalcy Editorial TeamLast reviewed About us
Gestational age counts from the first day of the last menstrual period rather than from conception, which is why it is roughly two weeks longer than the true fetal age. This calculator converts the time since your last period into weeks and days, adjusts for cycle length, names the trimester and estimates the due date. The result is an estimate until an early ultrasound confirms dating.
Weeks and days pregnant from the last menstrual period.
| Use 28 if you are unsure. |
GA (days) = days since LMP − (cycle − 28) EDD = LMP + 280 + (cycle − 28) days
A 28-day cycle assumes ovulation around day 14. A longer cycle shifts ovulation later, so the estimated due date moves later by the same number of days and gestational age is slightly lower at a given date. First-trimester ultrasound is more accurate than LMP dating when the two disagree, especially with irregular cycles.
| Symbol | Meaning |
|---|---|
| LMP | First day of the last menstrual period. |
| Cycle | Typical cycle length in days; 28 is the default. |
| GA | Gestational age in weeks and days. |
| EDD | Estimated due date, about 40 weeks from the LMP. |
With a 28-day cycle the adjustment is zero, so gestational age is 70 days, or 10 weeks 0 days. The estimated due date is 280 days after the LMP. With a 35-day cycle the adjustment is 7 days, so gestational age is 63 days (9 weeks 0 days) and the due date moves 7 days later.
Gestational age is used throughout pregnancy: screening windows, growth measurements, scheduling scans and decisions near term all depend on it. A due date that is a week off can move a screening test outside its valid window or make a normally grown baby look small. That is why clinicians prefer an early ultrasound when there is any doubt about the LMP or cycle length. The LMP method is simple and works well for people with regular 28-day cycles, but it assumes ovulation on day 14. Stress, illness, breastfeeding and cycle irregularity can all shift ovulation. The calculator's cycle adjustment is a reasonable correction, not a substitute for a dating scan. Use it to understand where you are, and let your maternity team confirm the official dates.
Continue with the Pregnancy Weight Gain Calculator, Conception Date Calculator, or the hCG Calculator — or browse more gynecology & pregnancy tools and all health calculators.
Also see the Ovulation Calculator and the Pregnancy Due Date Calculator.
The exact date of conception is rarely known, while the first day of the last period is easier to report. Counting from the LMP gives a consistent convention, even though fertilisation usually happens about two weeks later.
Pregnancy dating and fertility estimates assume regular cycles and typical ovulation. They cannot confirm pregnancy, viability or gestational age, and they do not replace ultrasound or prenatal care. Seek urgent care for pain, bleeding or reduced fetal movement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
Content updated: September 2026