Gestational age is the standard clock of pregnancy. It is counted in completed weeks and days from the first day of the last menstrual period (LMP), and nearly every prenatal milestone — screening windows, growth charts, the definition of term — is tied to it. This calculator works out gestational age on any date you choose and lists when key gestational ages will be reached.
How to use the gestational age calculator
- Pick how the pregnancy is dated: the first day of the last period, a known due date, an ultrasound result, or a conception date.
- Enter the matching information. For an ultrasound, enter the scan date and the gestational age the scan reported in weeks and days.
- Set Gestational age on to today or any other date, such as a future appointment.
- Read the weeks-and-days result, the fetal age, trimester and term status, and the table of upcoming dates.
How gestational age is calculated
Every method first finds a “dating LMP” — the date 280 days before the due date — and then counts forward:
For an ultrasound, the dating LMP is the scan date minus the gestational age measured at the scan; for a conception date, it is conception minus 14 days. Fetal age is simply gestational age minus 14 days.
Worked example
Last period began March 2, 2026. On June 30, 2026, 120 days have passed: 120 ÷ 7 = 17 weeks remainder 1, so gestational age is 17w 1d.
Fetal age is 15w 1d, the pregnancy is in the second trimester, and the due date is March 2 + 280 days = December 7, 2026.
An ultrasound on May 26, 2026 measuring 11w 5d (82 days) puts the dating LMP at March 5 and the due date at December 10 — three days later than the LMP date.
Under the ACOG rules below, a 3-day difference at 11 weeks is too small to change the due date, so the LMP-based date of December 7 would stand.
Gestational age versus fetal age
| Event | Gestational age | Fetal age |
|---|---|---|
| First day of last period | 0w 0d | — |
| Ovulation and conception | about 2w 0d | 0w 0d |
| Missed period | about 4w 0d | about 2w 0d |
| End of the embryonic period | 10w 0d | 8w 0d |
| Due date | 40w 0d | 38w 0d |
Gestational age is the convention in prenatal care, while fetal age is common in embryology texts and in describing IVF pregnancies.
When an ultrasound changes the due date
ACOG, the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine recommend redating a pregnancy when the first ultrasound disagrees with the LMP date by more than a set margin, which widens as pregnancy progresses:
| Gestational age at first scan | Redate if the difference is more than |
|---|---|
| Up to 8w 6d | 5 days |
| 9w 0d – 13w 6d | 7 days |
| 14w 0d – 15w 6d | 7 days |
| 16w 0d – 21w 6d | 10 days |
| 22w 0d – 27w 6d | 14 days |
| 28w 0d and later | 21 days |
Once set, the due date is normally not changed by later scans. If you have an ultrasound-confirmed due date, choose Known due date for the most consistent answer.
Why gestational age matters
Many tests are only valid in specific windows: nuchal translucency screening at 11w 0d to 13w 6d, the anatomy scan at 18 to 22 weeks, glucose screening at 24 to 28 weeks, and group B strep testing at 36 to 37 weeks. Birth timing is described the same way — before 37w 0d is preterm and 39w 0d to 40w 6d is full term. The table under your result shows the calendar dates for these points. For the full week-by-week timeline, use the pregnancy due date calculator.
For information only — this is not medical advice. Decisions about prenatal testing and delivery timing should follow the dating set by your healthcare professional, which may differ from these results.
Frequently asked questions
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period, while fetal (or embryonic) age is counted from conception. Because conception usually happens about two weeks after that period begins, fetal age is roughly two weeks less than gestational age.
How is gestational age written?
Clinicians write completed weeks plus extra days, such as 24w 3d or 24 3/7 weeks. Both mean 24 full weeks and 3 days, so the pregnancy is in its 25th week.
Which is more accurate, my last period or an ultrasound?
A first-trimester ultrasound that measures the embryo's crown-rump length is the most accurate standard method, typically within about 5 to 7 days. Period-based dating depends on remembering the date and on ovulating around day 14, so it is less precise when cycles are irregular.
How many months pregnant is a given number of weeks?
Months are an imprecise unit for pregnancy because they vary in length. Dividing weeks by about 4.35 gives calendar months, so 20 weeks is roughly 4.6 months. Clinicians use weeks and days for that reason.
Does a late ultrasound give an accurate gestational age?
Less so. Babies grow at different rates as pregnancy advances, so third-trimester measurements can be off by two to three weeks. That is why later scans only change the due date when they disagree with earlier dating by a wide margin.