Due date calculator

Providers date a pregnancy from the first day of your last menstrual period (LMP), not from conception. The American College of Obstetricians and Gynecologists (ACOG) puts the estimated due date at LMP + 280 days, which is exactly 40 weeks. This is Naegele's rule.

That rule assumes a 28-day cycle with ovulation around day 14. If your cycles are longer or shorter, ovulation probably came later or earlier, so the calculator shifts every date by the difference. You can enter a cycle length from 20 to 45 days.

Worked example: your last period started on 01/10/2026 and your cycle is 28 days.

• Due date: 01/10/2026 + 280 days = 10/17/2026
• 37 weeks (early term): 09/26/2026
• 42 weeks: 10/31/2026

With a 31-day cycle, everything moves 3 days later, so the due date becomes 10/20/2026.

Not everyone knows the date of their last period, so the calculator offers five starting points. Each one is converted to the same LMP-equivalent day 0, and the due date is always that day plus 280 days.

You know	What you enter	How the due date is set
Last period	First day of LMP, cycle length	LMP + 280 days, shifted for cycle length
Conception date	Day you conceived or ovulated	Conception + 266 days (38 weeks)
IVF transfer	Transfer date and embryo age (day 3, 5 or 6), or egg retrieval / IUI date	Transfer + 266 − embryo age, so day 5 = +261, day 3 = +263
Ultrasound	Scan date and the age measured (weeks + days)	Scan date − measured age + 280 days
Known due date	The date your provider gave you	Shows how far along you are and your timeline

For IVF, the IVF due date calculator explains the embryo-age formula in more detail. If you only know when you had sex, the conception calculator shows the likely window.

If you pick Last period, you can tick I've had an ultrasound and enter the scan date and the gestational age it measured. The calculator then applies ACOG's redating rule from Committee Opinion 700: if the two estimates differ by more than the threshold for the week of the scan, the ultrasound date is used.

Ultrasound age at the scan	Redate if the difference is more than
up to 8+6 weeks	5 days
9+0 to 13+6 weeks	7 days
14+0 to 15+6 weeks	7 days
16+0 to 21+6 weeks	10 days
22+0 to 27+6 weeks	14 days
28+0 weeks or later	21 days

Example: LMP 01/10/2026. A scan on 03/10/2026 measures 7 weeks 4 days, while your LMP puts you at 8 weeks 3 days. The gap is 6 days, which is over the 5-day threshold, so the ultrasound due date applies: 10/23/2026. If the scan had measured 8 weeks 0 days, the 3-day gap would keep the LMP date. Your provider always makes the final call.

• Estimated due date with the weekday, plus days to go (or days past the date).
• How far along you are today in weeks + days, the way providers write it: 10+3 means 10 weeks and 3 days.
• Trimester: first through 13+6 weeks, second from 14+0, third from 28+0, as ACOG defines them.
• Stage, using ACOG's term definitions: early term 37+0 to 38+6, full term 39+0 to 40+6, late term 41+0 to 41+6, postterm from 42+0.
• Pregnancy dates: estimated conception, the LMP-equivalent date, 37 weeks, 40 weeks and 42 weeks.
• Key-date timeline: when a home test is usually reliable, first-trimester screening (11–13 weeks), the anatomy scan (18–22 weeks), first movements (often 16–24 weeks), glucose screening (24–28 weeks) and the term milestones. Past dates are checked off and the current one is highlighted.

The .ics button downloads these dates as a calendar file you can open in Google Calendar, Apple Calendar or Outlook. A link takes you straight to your current week in pregnancy week by week. Everything is calculated in your browser; your dates are not sent to a server or saved.

A due date is an estimate, not a deadline. Most babies are born between 37 and 42 weeks, and only a few arrive on the exact day. ACOG considers a first-trimester ultrasound the most accurate way to confirm or set the due date, which is why the calculator lets you compare the two.

The LMP method is less reliable if you have irregular cycles or aren't sure of the date. In that case, use the ultrasound option or the due date your provider gave you.

This calculator is informational and is not a substitute for medical advice. Call your OB-GYN or midwife, or go to labor and delivery, right away if you have vaginal bleeding, your water breaks, you have constant severe pain, or your baby is moving less than usual. In an emergency, call 911.