How it is calculated
Pregnancy is dated from the first day of the last menstrual period (not conception), so counting starts about two weeks before actual conception:
Weeks = (today − LMP) ÷ 7| Month | Weeks |
|---|---|
| 1 | 1 – 4 |
| 2 | 5 – 8 |
| 3 | 9 – 13 |
| 4 | 14 – 17 |
| 5 | 18 – 22 |
| 6 | 23 – 27 |
| 7 | 28 – 31 |
| 8 | 32 – 35 |
| 9 | 36 – 40 |
Worked example
If 150 days have passed since your last period began, you are 21 weeks and 3 days pregnant — in month five and the second trimester.
Frequently asked questions
Why does my doctor give a different week?
Your doctor may adjust dating from an early ultrasound, which is the most accurate.
“Week 20” vs “20 weeks”?
Both are used; “20 weeks and 3 days” means you are in your 21st week.
When is a baby full term?
From 37 weeks; birth between 37 and 42 weeks is considered term.