Gestational Age Calculator
Find out how many weeks and days pregnant you are using any dating method: last menstrual period (LMP), conception date, IVF embryo transfer, an ultrasound estimate, or a known due date. You get the gestational age, trimester, fetal (conception) age, estimated conception and due dates, percent complete, and a full timeline of milestone dates.
Formula
Worked example
If your LMP was January 1 with a 28-day cycle and today is March 26 (84 days later), gestational age is 84 = 7 x 12 + 0, so 12 wk 0 d. Conception is about January 15, the estimated due date is 280 days after January 1 (October 8), and fetal age is 84 - 14 = 70 days, or 10 wk 0 d.
How gestational age is measured
Gestational age is the standard way clinicians describe how far along a pregnancy is. By convention it is counted from the first day of the last menstrual period (LMP), not from conception, because the LMP is a date most people can recall while the exact moment of fertilization is rarely known. This adds roughly two weeks compared with the true age of the embryo, since ovulation and conception typically occur about 14 days after the LMP in an average 28-day cycle. The figure is expressed as completed weeks plus extra days, for example "24 wk 3 d" means twenty-four full weeks and three days have elapsed. This calculator also reports the fetal age, the true age since conception, which is about two weeks less.
Five ways to date a pregnancy
You can start from whichever date you know best, and every method resolves to the same 280-day timeline. The LMP method counts forward from your last period and lets you adjust for a cycle that is longer or shorter than 28 days, shifting the due date by (cycle minus 28) days. The conception or ovulation method subtracts about two weeks to find the menstrual anchor. The IVF method uses the embryo transfer date and whether the embryo was a day 3, day 5, or day 6 transfer, which pins dating very precisely. The ultrasound method takes the weeks and days a scan measured on a given day and works backward. The known due date method reverse solves: enter an EDD your provider already set and the calculator tells you today's gestational age and the implied conception date.
Trimesters, milestones and the due date
A full-term pregnancy lasts about 40 weeks, or 280 days, from the LMP, and that 40-week mark is the estimated due date this calculator reports. The American College of Obstetricians and Gynecologists divides the journey into three trimesters: the first runs through 13 weeks 6 days, the second from 14 weeks through 27 weeks 6 days, and the third from 28 weeks until birth. Babies born between 37 and 42 weeks are considered term, with 39 to 40 weeks regarded as the optimal full-term window, and 24 weeks marks the usual viability milestone. The milestone table turns these gestational ages into actual calendar dates for your pregnancy so you can plan scans and appointments.
Why dating can change
LMP-based dating assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter, or irregular, the true gestational age can differ from this estimate by several days even after the cycle adjustment. For this reason a first-trimester ultrasound, which measures the embryo directly, is considered the most accurate method, and obstetricians will often revise the due date if an early scan disagrees with the LMP by more than five to seven days. Always treat this tool as a starting estimate and confirm your dates with a prenatal provider.
Pregnancy timeline by gestational age
| Stage | Gestational age | Notes |
|---|---|---|
| First trimester | 0 wk 0 d, 13 wk 6 d | Conception through week 13 |
| Second trimester | 14 wk 0 d, 27 wk 6 d | Often the most comfortable weeks |
| Viability | 24 wk 0 d | Survival outside the womb becomes possible with care |
| Third trimester | 28 wk 0 d, 40 wk+ | Final stretch to delivery |
| Full term | 39 wk 0 d, 40 wk 6 d | Optimal delivery window |
| Estimated due date | 40 wk 0 d | 280 days after LMP |
Trimesters are defined by completed weeks of gestation, counted from the first day of the last menstrual period (LMP).
Frequently asked questions
Is gestational age the same as the age of my baby?
No. Gestational age is counted from the first day of your last menstrual period, which is about two weeks before conception actually occurs. The true age of the embryo or fetus, called fetal or conceptional age, is therefore roughly two weeks less than the gestational age. This calculator shows both figures.
How does cycle length change the result?
Standard dating assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation shifts, so the calculator moves the due date by (your cycle length minus 28) days when you use the LMP method. A 35-day cycle, for example, pushes the due date about a week later.
Can I calculate from an IVF transfer or a known due date?
Yes. Pick the IVF method and enter the transfer date and whether it was a day 3, day 5, or day 6 embryo for very precise dating. Or pick the known due date method to reverse solve: enter the EDD your provider set and the calculator returns today's gestational age, trimester, and estimated conception date.
What if my menstrual cycle is irregular?
LMP dating, even with the cycle adjustment, assumes reasonably regular ovulation, so irregular cycles make it less reliable. In that case an early ultrasound gives a more accurate gestational age, and your provider may adjust your due date. Use the ultrasound method here to mirror their dating, and confirm with a clinician.