Pregnancy tools
How many weeks pregnant am I?
See exactly how many weeks and days pregnant you are today and which trimester you are in. You can also check how far along you will be on any date, such as a holiday, wedding or flight.
Your weeks appear hereEnter the first day of your last period or your due date.
Comfort for the weeks ahead
How pregnancy weeks are counted
Pregnancy weeks are counted from the first day of your last period, so for the first two weeks you are not actually pregnant yet. When someone says they are 12 weeks, they mean 12 completed weeks since that day. At 12 weeks and 3 days you are in your 13th week.
If you already have a due date from a scan, choose Due date. The calculator counts back 280 days from it, which matches how your midwife counts.
Trimesters at a glance
- First trimester: up to the end of week 12. The booking appointment and the dating scan.
- Second trimester: week 13 to the end of week 27. The 20-week scan, and many people start to feel their baby move.
- Third trimester: week 28 until birth. Appointments come closer together, and the advice is to go to sleep on your side.
Planning around a date
Use Check a date to see how pregnant you will be on a future day. It helps when booking travel: many airlines ask for a letter from your midwife or GP in the third trimester and have their own cut-off dates, so check with your airline. It also helps you plan work: you need to tell your employer no later than the end of the 15th week before the week your baby is due. The maternity leave calculator gives you that exact date.
Questions
Am I 12 weeks pregnant or in week 12?
If you are 12 weeks and some days, you have completed 12 weeks and are in week 13. The calculator shows both.
Why does my app give a different number?
Apps can count from your period, your conception date or a scan, and some round to the nearest week. After your dating scan, choose Due date and enter the date your midwife gave you.
How many weeks is a full-term pregnancy?
Pregnancy usually lasts between 37 and 42 weeks. Week 40 is your estimated due date, not a deadline.
When does the third trimester start?
At 28 weeks. From then, UK guidance is to go to sleep on your side, including for daytime naps.
How many months pregnant is 20 weeks?
About four and a half months. Months do not divide neatly into weeks, which is why midwives count in weeks and days.
Sources
Last reviewed 13 October 2026. These tools give estimates for information only and are not medical or legal advice. Your midwife, GP or fertility clinic confirms your dates; your employer and gov.uk confirm your leave and pay.