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Pregnancy Due Date Calculator

Find out when your baby is due using your last menstrual period or conception date. We also show how far along you are today and which trimester you’re in.

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How due dates are calculated

The standard formula in obstetrics is Naegele’s rule: due date = first day of last menstrual period (LMP) + 280 days. That works out to 40 weeks of pregnancy, which sounds odd because conception happens about two weeks after LMP — pregnancy is counted from the start of your last period, not from conception.

If you happen to know the exact conception date — for example, from tracking ovulation closely or IVF — use the conception-date option instead. We add 266 days from conception, which gets you to the same due date.

The three trimesters at a glance

  • First trimester (weeks 1–12): heartbeat develops, organs form, early symptoms like nausea and fatigue peak.
  • Second trimester (weeks 13–27): you’ll usually feel the baby move (“quickening”), anatomy ultrasound around week 20, energy often improves.
  • Third trimester (weeks 28–40+): baby grows fastest, more frequent provider visits, prep for birth.

Why the due date can shift

Naegele’s rule assumes a textbook 28-day cycle with ovulation on day 14. Real cycles vary. Reasons your provider may adjust the due date:

  • Early dating ultrasound (before week 14) measures the baby’s size directly — usually overrides the LMP date.
  • Your typical cycle is longer or shorter than 28 days, which moves your ovulation day earlier or later.
  • You weren’t sure of the exact first day of your last period, or had spotting that was confused with a period.

Frequently asked questions

How is the due date calculated?
We use Naegele’s rule, the standard formula in obstetrics: due date = first day of last menstrual period + 280 days. That’s the same as adding 9 months and 7 days. If you know your conception date instead, we add 266 days.
How accurate is the due date?
It’s an estimate. Only about 5% of babies are born on their exact due date, but most arrive within two weeks of it — between 38 and 42 weeks of pregnancy. An early ultrasound (before 14 weeks) gives a more accurate due date than the LMP formula because it measures the baby’s actual size.
What are the three trimesters?
First trimester: weeks 1–12. Most early pregnancy symptoms appear here. Second trimester: weeks 13–27. Often the most comfortable stretch. Third trimester: weeks 28 to birth. Final growth and preparation for delivery.
What if I have irregular periods?
Naegele’s rule assumes a regular 28-day cycle and ovulation on day 14. If your cycles are longer, shorter, or irregular, the LMP-based due date can be off by a week or more. An early dating ultrasound is the most reliable way to confirm your due date.
Can I use this if I had IVF?
If you know the date of egg retrieval or transfer, use the conception-date option and enter the day of retrieval (or transfer date minus 3 or 5 days depending on the embryo stage). Your fertility clinic will normally calculate this for you with the exact protocol.

Not sure yet if you’re pregnant?

If you’re here with a hunch but haven’t confirmed it yet, our guide on PMS vs pregnancy: 9 ways to tell the difference covers the most reliable early signs to look for and when a test will be accurate.

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