How to use
- 1Choose how to calculate: last period, conception date or IVF transfer.
- 2Enter the date, plus your cycle length (for last period) or embryo age (day 3 or day 5 for IVF).
- 3Read your estimated due date, how far along you are, the trimester and progress.
- 4Check the key dates for scans and tests, marked as past, now or upcoming.
How the due date is calculated
The standard method is Naegele’s rule: the first day of your last menstrual period (LMP) plus 280 days, or 40 weeks. If your cycle is longer or shorter than 28 days, the date is shifted by the difference. From a known conception date the due date is 266 days later; after IVF it is 263 days after a day-3 embryo transfer and 261 days after a day-5 (blastocyst) transfer, as described in ACOG guidance. If you had a first-trimester dating ultrasound, your clinician may adjust the due date to it.
Weeks, trimesters and term
Pregnancy is counted from the LMP, so at conception you are already about two weeks pregnant. The first trimester runs to 13 weeks 6 days, the second from 14 to 27 weeks 6 days and the third from 28 weeks. ACOG defines 37 weeks as early term, 39 weeks as full term and 42 weeks as post-term. Only a small share of babies arrive exactly on the due date; most are born within about two weeks either side of it.
Key scan and test dates
The calculator lists typical windows: the first or dating ultrasound (about 8–13 weeks), the nuchal translucency (NT) scan and combined screening (11–13+6 weeks), the anatomy scan (18–22 weeks), glucose screening (24–28 weeks) and the group B strep test (36–37+6 weeks), plus 37, 39, 40 and 42 weeks. These windows follow ACOG and NHS guidance; your own schedule may differ by country and provider.
Not medical advice
This calculator is for information only and is not medical advice. Follow your doctor’s or midwife’s guidance for all pregnancy care, and seek care immediately for bleeding, severe pain or reduced baby movements. Dates are calculated in your browser and nothing is stored or sent.
Frequently asked questions
How is my due date calculated?
By default, the first day of your last period plus 280 days (40 weeks), adjusted for your cycle length. Conception date and IVF transfer dates can be used instead.
How many weeks pregnant am I?
Enter the first day of your last period; the calculator shows your gestational age in weeks and days as of today, or any date you choose.
How accurate is a due date?
It is an estimate. Only about 1 in 20 babies is born on the exact date; a first-trimester ultrasound gives the most accurate dating.
How do I calculate an IVF due date?
Choose “IVF transfer”, enter the transfer date and select a day-3 or day-5 embryo. The due date is 263 or 261 days after the transfer.
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