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
Common questions
How many weeks pregnant am I?
You count pregnancy in weeks and days from the first day of your last period, the way the NHS dates it. So if it has been 70 days since your last period, you are 10 weeks pregnant. A full-term pregnancy is about 40 weeks (280 days), and if you know your due date instead, the tool works the last period back as the due date minus 280 days.
Which trimester am I in?
The first trimester runs to the end of week 12, the second from week 13 to week 26, and the third from week 27 to birth. The tool shows your trimester and how far through the 40 weeks you are.
How does the sibling age gap tool decide the school year?
It uses the England cutoff of 1 September: two children share a school year only if both birthdays fall inside the same 1 September to 31 August window. A child born on or before 31 August starts Reception the September after they turn 4. Scotland, Wales and Northern Ireland use different cutoffs, so confirm with your local authority.
Are these tools medical advice?
No. The pregnancy tools are general information, not personalised medical advice. Your midwife dates your pregnancy precisely at your dating scan. Ask your midwife or GP about your own pregnancy and any concerns.
Email me when this updates
No account, no spam. Just a short note when we refresh the pregnancy and family tools or add a related tool. Unsubscribe any time by replying.
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