Pregnancy Due Date Calculator

What this tool does

The Pregnancy Due Date Calculator applies Naegele's rule, adjusted for your cycle length, or works from a conception date, a day-3 or day-5 IVF transfer or an ultrasound dating scan. It shows the estimated due date, how far along you are today, the current trimester and a dated list of scans and milestones.

Pregnancy is dated from the first day of the last menstrual period (LMP), not from conception, because the LMP is usually the date people actually know. A typical pregnancy lasts 280 days, or 40 weeks, from the LMP. That convention means you are already "two weeks pregnant" around the time of conception, which surprises many first-time parents.

This calculator uses Naegele's rule (LMP + 280 days) and shifts the date when your cycle is longer or shorter than 28 days, since ovulation moves with cycle length. If you know when you conceived it adds 266 days; for IVF it adds 266 days minus the embryo's age at transfer (263 for a day-3 embryo, 261 for a day-5 blastocyst). With an ultrasound it back-dates from the gestational age measured at the scan. All dates are worked out on your device and nothing is stored online.

How to use Pregnancy Due Date Calculator

1. Choose what to calculate from: last period, conception date, IVF transfer or ultrasound scan.
2. Enter the date in the Date field.
3. For the LMP method, set your average cycle length; for IVF, pick a day-3 or day-5 embryo; for a scan, enter the weeks and days reported.
4. Read the estimated due date, gestational age today and days remaining.
5. Scroll through the milestone table for dates of the dating scan window, anatomy scan, third trimester and term.
6. Download the milestones as CSV to add them to a calendar.

Tips for better results

• If an early ultrasound (before about 14 weeks) disagrees with your LMP date by more than about a week, clinicians usually adopt the scan date.
• Note the time window for each screening test, not only the single date, so you can book appointments in advance.
• Irregular cycles make LMP dating unreliable; a dating scan is the better reference.

Common use cases

• Working out how many weeks pregnant you are today.
• Planning maternity leave and telling an employer the expected week of childbirth.
• Checking the transfer-based due date after an IVF cycle.
• Timing antenatal scans and glucose screening appointments.

Common mistakes to avoid

• Entering the date of a positive test or the last day of the period instead of the first day.
• Treating the due date as the expected delivery day; only about 4 to 5 percent of babies arrive on it.
• Forgetting to set a long or short cycle length, which can shift the date by a week or more.
• Using LMP dating after an IVF transfer, when the transfer date is far more precise.