Find out exactly how many weeks and days pregnant you are, your trimester, and what’s coming up next in your pregnancy.
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How pregnancy weeks are counted
Pregnancy is dated from the first day of your last period, not from conception, so it lasts about 40 weeks (280 days). If your cycle is longer or shorter than 28 days, the calculator adjusts for it. Your dating scan may then fine-tune your due date.
| Trimester | Weeks |
|---|---|
| First | 0 to 12 weeks |
| Second | 13 to 26 weeks |
| Third | 27 weeks to birth |
Key dates (UK NHS care)
| Week | What happens |
|---|---|
| By 10 | Booking appointment with your midwife |
| 10–14 | Dating scan |
| 16 onwards | Whooping cough vaccine (ideally by 32 weeks) |
| 18–21 | Anomaly scan |
| 20 | MATB1 certificate available for maternity pay |
| 25 | Tell your employer by the 15th week before your due date |
| 28 onwards | RSV vaccine offered |
| 40 | Due date |
Example
If your last period started 140 days ago, you’re 20 weeks 0 days pregnant, in the second trimester, with 140 days to go.
Frequently asked questions
How many weeks pregnant am I?
Count from the first day of your last period. The calculator does this for you, adjusting for your cycle length.
When does the second trimester start?
At 13 weeks. The third trimester starts at 27 weeks.
Is my due date exact?
No. It’s an estimate; only around 1 in 20 babies are born on their due date.
Can I use my due date instead?
Yes. Choose ‘Due date’ and enter the date from your scan.
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