Pregnancy Calculator
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How to use the pregnancy calculator
- Choose how to date the pregnancy. Most people start with the first day of their last period; if you have had a dating scan, use the scan instead.
- Enter the date (and your usual cycle length, the gestational age on the scan report, or the embryo age for IVF).
- Leave "How far along on" as today, or pick another date to see where you will be on a holiday, a scan or a birthday.
- Read how far along you are, your trimester, the due date and the timeline. "Now" marks a window you are in today.
How gestational age is calculated
LMP-equivalent (cycle length c) = LMP + (c − 28) days
from an ultrasound = scan date − gestational age at the scan
from conception = conception date − 14 days
from IVF = transfer date − 19 days (day-5 embryo) or − 17 days (day 3)
due date = LMP-equivalent + 280 days
Pregnancy is counted from the first day of the last period, about two weeks before conception, so at conception you are already "2 weeks pregnant". Every method is converted to this common starting point. A cycle longer than 28 days usually means later ovulation, so the start moves later by the same number of days. For IVF, ACOG sets the due date 261 days after a day-5 embryo transfer or 263 days after a day-3 transfer, which is the same as counting back 19 or 17 days to the starting point. Trimesters follow ACOG's definitions: the first runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days.
Worked example
Your last period started on Jan 1, 2026 and you have 28-day cycles. On Apr 10, 2026, 99 days have passed, which is 14 weeks 1 day: you are in week 15 of pregnancy and in the second trimester. The estimated due date is Thursday, October 8, 2026. If a scan on Feb 20, 2026 had measured 7 weeks 2 days, the due date would be Oct 7, 2026 instead; a day-5 embryo transferred on Jan 20, 2026 gives Oct 8, 2026.
Pregnancy timeline
Milestones with ACOG's usual timing, and the dates they fall on for a last period starting Jan 1, 2026.
| Milestone | Gestational age | Example dates |
|---|---|---|
| Estimated conception | About 2 weeks | Jan 15, 2026 |
| Expected period missed | 4 weeks | Jan 29, 2026 |
| First trimester ends | 13 weeks 6 days | Apr 8, 2026 |
| Standard (anatomy) ultrasound | 18 to 22 weeks | May 7 – Jun 10, 2026 |
| Glucose screening for gestational diabetes | 24 to 28 weeks | Jun 18 – Jul 22, 2026 |
| Third trimester begins | 28 weeks 0 days | Jul 16, 2026 |
| Group B strep test | 36 weeks 0 days to 37 weeks 6 days | Sep 10–23, 2026 |
| Early term begins | 37 weeks 0 days | Sep 17, 2026 |
| Full term begins | 39 weeks 0 days | Oct 1, 2026 |
| Estimated due date | 40 weeks 0 days | Oct 8, 2026 |
| Late term begins | 41 weeks 0 days | Oct 15, 2026 |
| Post-term begins | 42 weeks 0 days | Oct 22, 2026 |
ACOG recommends at least one standard ultrasound, usually at 18 to 22 weeks, to check the baby's anatomy. Screening for gestational diabetes is usually done between 24 and 28 weeks, and the group B strep swab at 36 weeks 0 days to 37 weeks 6 days. Your own schedule may differ: follow your care team.
Reading your result
- "14 weeks 1 day" is week 15. Gestational age counts completed weeks, so once 14 full weeks have passed you are in your 15th week.
- An early scan beats your period date. ACOG calls a first-trimester ultrasound the most accurate way to date a pregnancy, and your due date may be changed after one. Use the scan method once you have it.
- Irregular cycles make period dates unreliable. If your cycle length varies a lot, or you are not sure when your last period started, ACOG advises dating by ultrasound instead.
- Few babies arrive on the due date. Only about 1 in 20 do, according to ACOG. Full term runs from 39 weeks 0 days to 40 weeks 6 days; see the due date calculator for the whole term window.
All of these dates are estimates. This is not medical advice; talk to your doctor or midwife. To work out when you probably conceived, use the pregnancy conception calculator, and to track weight gain, the pregnancy weight gain calculator.
Sources
- ACOG, AIUM and SMFM, Committee Opinion 700: Methods for Estimating the Due Date (2017).
- ACOG FAQs: How Your Fetus Grows During Pregnancy (January 2024), Ultrasound Exams (January 2024) and Routine Tests During Pregnancy (April 2024).
- ACOG Committee Opinion 797, Prevention of Group B Streptococcal Early-Onset Disease in Newborns (2020); ACOG and SMFM Committee Opinion 579, Definition of Term Pregnancy (2013).
Frequently asked questions
How do I work out how many weeks pregnant I am?
Count the days from the first day of your last period and divide by 7. If your last period started on January 1, 2026, then on April 10, 2026 you are 99 days, or 14 weeks 1 day, pregnant, which is week 15 and the second trimester.
Why does pregnancy start before conception?
By convention, gestational age is counted from the first day of the last period, because that date is usually known and conception is not. Conception typically happens about two weeks later, so you are already about 2 weeks pregnant when it occurs.
When does each trimester start?
ACOG defines the first trimester as running to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days. For a last period on January 1, 2026, the second trimester starts on April 9 and the third on July 16.
Which is more accurate, my last period or an ultrasound?
ACOG calls a first-trimester ultrasound the most accurate way to date a pregnancy: up to 13 weeks 6 days it is accurate to about 5 to 7 days. Your care provider may change your due date after a scan, and you should then use the scan date in this calculator.
How is the due date set after IVF?
ACOG uses the embryo age and transfer date: the due date is 261 days after a day-5 embryo transfer and 263 days after a day-3 transfer. A day-5 transfer on January 20, 2026 gives a due date of October 8, 2026.