How pregnancy weeks are counted
Gestational age counts completed weeks and days from the first day of the last menstrual period, even though conception usually occurs later. A result of 20+3 means 20 completed weeks and three additional days.
Every due date represents 40+0. The calculator counts backward or forward from that date using calendar days, so daylight-saving clock changes do not alter the result.
Sources: American College of Obstetricians and Gynecologists, NHS Best Start in Life.
Which date should you use?
Use the estimated due date documented by your care team whenever you have one. First-trimester ultrasound is generally the most accurate way to establish or confirm gestational age, and a clinician may replace an earlier period-based estimate.
If you do not have an established due date yet, use the separate due-date calculator for a provisional estimate. A period-based estimate cannot account for uncertain dates, a dating scan, or fertility treatment without additional information.
Sources: American College of Obstetricians and Gynecologists, NHS.
Where the trimester labels change
This tool follows the American College of Obstetricians and Gynecologists boundaries: the first trimester runs through 13+6, the second from 14+0 through 27+6, and the third begins at 28+0.
Some services and publications group weeks differently. For appointments, screening, medication, or urgent decisions, use the exact weeks and days and follow your local care team rather than relying on a trimester label.
Sources: American College of Obstetricians and Gynecologists.
A due date is an estimate
The estimated due date is a reference point, not a promise about the day labor will begin. ACOG divides the weeks around it into early term, full term, late term, and postterm categories.
Do not change a recorded due date each time a later scan produces a slightly different measurement. Dating decisions belong with the clinician who can see the full record.
Sources: American College of Obstetricians and Gynecologists, American College of Obstetricians and Gynecologists.
What this result does not tell you
Pregnancy week does not predict development on an exact day, confirm that a pregnancy is progressing normally, or decide when a symptom needs assessment. Milestones describe broad patterns with normal variation.
Contact your maternity team whenever you are worried. Reduced, absent, or changed movement needs immediate contact and should never wait for a calculator result.
Sources: NHS.
Sources
- 01How Your Fetus Grows During PregnancyAmerican College of Obstetricians and Gynecologists
- 02Methods for Estimating the Due DateAmerican College of Obstetricians and Gynecologists · May 2017; reaffirmed 2025
- 03Definition of Term PregnancyAmerican College of Obstetricians and Gynecologists
- 04Week-by-week guide to pregnancyNHS Best Start in Life
- 05Pregnancy due date calculatorNHS · Last reviewed August 2024
- 06Your baby's movementsNHS · Last reviewed July 2024