From the first name on the list to the first day of school.
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UK baby tools and data that show their working
BabyData is a UK baby site anchored on baby names and built on official data. The tools are free, instant and transparent: due date, ovulation, how many weeks pregnant you are, sibling age gaps and maternity pay, each one showing the rule it uses and citing the NHS or gov.uk source behind it. The data pages turn the ONS baby-names and births datasets into something you can actually read. No sign-up, no email. Pregnancy content is informational only, not medical advice, and is reviewed by our editorial team against NHS and gov.uk guidance.
Start with the question almost everyone asks first: when is the baby due. Enter your last period, conception date or IVF transfer and the estimate appears instantly, with the working a click away.
Due date calculator
Your estimated due date and how many weeks pregnant you are, by Naegele's rule (NHS).
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.
Where to next
The four desksNames and Family Data Desk, BabyData
BabyData's editorial desk builds and documents the tools, citing the underlying rule and the official UK dataset behind every number. Pregnancy-related tools are editorially reviewed against NHS and gov.uk guidance before publication.
Last reviewed: 12 June 2026