Divide by 4.345, not 4
A month is not four weeks. Averaged over a year it is 4.345 weeks (365.25 ÷ 12 ÷ 7). Dividing by 4 instead looks close but drifts: 40 weeks comes out as 10 months rather than the correct 9.2. Only a 28-day February is exactly four weeks.
Weeks to months at a glance
| Weeks | Months |
|---|---|
| 4 weeks | 0.92 |
| 6 weeks | 1.38 |
| 8 weeks | 1.84 |
| 12 weeks | 2.76 |
| 16 weeks | 3.68 |
| 20 weeks | 4.60 |
| 26 weeks | 5.98 |
| 40 weeks | 9.20 |
| 52 weeks | 11.96 |
For a rough figure in your head, multiply weeks by 0.23. Twelve weeks × 0.23 = 2.76 months.
Pregnancy counts months differently
Antenatal care groups weeks into fixed months rather than the calendar average, so the mapping below is what a midwife uses. It is why "9 months pregnant" actually means about 40 weeks.
| Month | Weeks | Trimester |
|---|---|---|
| 3 | 9–13 | First |
| 5 | 18–22 | Second |
| 6 | 23–27 | Second |
| 9 | 36–40 | Third |
These weeks are measured from the last menstrual period. Full term is 39 to 40 weeks, so the "ninth month" runs a little past a calendar nine.
Common questions
How many weeks are in a month?
On average 4.345 weeks, worked out as 365.25 days divided by 12 months and then by 7. Only February in a normal year is exactly 4 weeks; a 31-day month is about 4.43 weeks.
How many months is 12 weeks?
About 2.76 months (12 divided by 4.345). In pregnancy terms, 12 weeks marks the end of month three and the first trimester.
Why is pregnancy 40 weeks but only 9 months?
Because a month averages 4.345 weeks, not 4, so 40 divided by 4.345 is 9.2 months. Pregnancy is also counted from the last period, roughly two weeks before conception.
