Due date and ovulation
Your due date is estimated as 40 weeks (280 days) from the first day of your last period, the rule the NHS uses. BabyData works it out from your last period, conception date or IVF transfer, tells you how many weeks pregnant you are now, and estimates your fertile window. Each tool shows the exact rule it uses. These are estimates, not medical advice, and your midwife may adjust the date at your dating scan.
Due date calculator
Estimated due date and weeks pregnant from LMP, conception or IVF transfer.
How this is worked out
Due dates use Naegele's rule, the standard the NHS and most clinicians use. A pregnancy is dated as roughly 40 weeks (280 days) from the first day of your last menstrual period (LMP):
Due date = LMP + 280 days
From a known conception date, the count is 266 days (38 weeks), because ovulation is about 14 days after the LMP. For IVF the embryo age is already known, so the date is the transfer date plus 266 days minus the embryo age (3 or 5 days).
How many weeks pregnant you are now is counted from the LMP (or the equivalent LMP worked back from conception or transfer), as the NHS does.
An estimate only, not a medical diagnosis. Only about 1 in 20 babies arrive on the exact due date. Your midwife may revise the date after your dating scan. Editorially reviewed against NHS and gov.uk guidance.
How many weeks pregnant am I?
Weeks, days, trimester and how far through the 40 weeks you are.
How this is worked out
Pregnancy is dated in weeks plus days from the first day of your last period, the way the NHS counts it. A full-term pregnancy is about 40 weeks (280 days).
Weeks pregnant = (today − LMP) ÷ 7
If you know your due date instead, the LMP is worked back as the due date minus 280 days. The first trimester runs to the end of week 12, the second from week 13 to 26, and the third from week 27 to birth.
Informational only, not medical advice. Your midwife dates your pregnancy precisely at your dating scan. Editorially reviewed against NHS and gov.uk guidance.
Ovulation and fertile window
Your estimated ovulation day and most fertile days, from your cycle.
How this is worked out
Ovulation usually happens about 14 days before your next period, because the luteal phase (from ovulation to the period) stays roughly constant while cycle length varies:
Ovulation day = cycle length − 14
The fertile window is the day of ovulation plus the 5 days before it, because sperm can survive in the body for up to about 5 days, as the NHS notes. Your next period is estimated as the first day of your last period plus your cycle length.
An estimate only. Cycles vary month to month, so this is a guide rather than a guarantee. Ovulation predictor kits or speaking to your GP can give more certainty.
More date tools
Common questions
How is my due date worked out?
Your due date is estimated as 280 days (40 weeks) from the first day of your last period, which is Naegele's rule and the method the NHS uses. From a known conception date the count is 266 days, and for IVF it is the transfer date plus 266 days minus the embryo age. Only about 1 in 20 babies arrive on the exact due date.
Why did my scan change my due date?
A dating scan measures the baby directly using the crown-rump length, which is a very accurate guide to gestational age in early pregnancy. A due date from your last period only estimates it, so the two can differ. NICE guidance says that when the scan and your last period disagree by more than a few days, the pregnancy is dated from the scan, and that scan date becomes your official due date.
How does the ovulation calculator find my fertile window?
It estimates ovulation as your cycle length minus 14 days, because the luteal phase stays roughly constant while cycle length varies. The fertile window is the day of ovulation plus the five days before it, since sperm can survive in the body for up to about five days, as the NHS notes. Cycles vary month to month, so treat it as a guide.
Are these tools medical advice?
No. They are general information, not personalised medical advice. Your midwife dates your pregnancy precisely at your dating scan and may revise your due date. Ask your midwife or GP about your own dates and any concerns.
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Editor, BabyData
Oliver Mackman is the named editor of BabyData and a director of Best Business Loans Ltd (Companies House 16833937), the company that operates the site. He is accountable for its editorial standards, sourcing and corrections, and requires every number to cite the underlying rule and the official UK dataset behind it, with pregnancy tools reviewed against NHS and gov.uk guidance before publication.
Last reviewed: 12 June 2026