Pregnancy tools
Pregnancy is counted in weeks and days from the first day of your last period, the way the NHS dates it, with the first trimester running to week 12, the second to week 26 and the third to birth. BabyData tells you how far along you are and works out the age gap between two children, including whether they share a UK school year. Pregnancy content here is informational only, not medical advice, and is reviewed by our editorial team against NHS and gov.uk guidance.
How many weeks pregnant am I?
Weeks, days, trimester and progress through the 40 weeks.
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.
Sibling age gap calculator
The gap between two children in years, months and days, plus whether they share an England school year.
How this is worked out
The age gap is the difference between the two dates, shown in years, months and days.
For school year we use the England cutoff of 1 September. A child whose birthday falls between 1 September and 31 August belongs to that school year, so a child born on or before 31 August starts Reception the September after their fourth birthday. Two children share a school year only if both birthdays sit inside the same 1 September to 31 August window.
School year window = 1 September to 31 August
School-year cutoffs differ in Scotland, Wales and Northern Ireland, so this England rule is a guide. Check with your local authority for admissions.
More pregnancy and family tools
Names 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