How is a due date calculated?
The standard method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). Pregnancy is counted from the LMP, not conception, which is why you're already considered "2 weeks pregnant" at the moment of conception.
The three trimesters
- First trimester: weeks 1–12
- Second trimester: weeks 13–27
- Third trimester: weeks 28–40
Pregnancy weeks to months
Weeks are counted from the first day of your last period, as doctors do. Months are approximate — a month is about 4.3 weeks, which is why a 40-week pregnancy is closer to 9.2 months than 9. "Weeks to go" counts to the 40-week due date.
| Weeks | ≈ Months | Trimester | Weeks to go |
|---|---|---|---|
| 4 | 0.9 | First | 36 |
| 5 | 1.2 | First | 35 |
| 6 | 1.4 | First | 34 |
| 7 | 1.6 | First | 33 |
| 8 | 1.8 | First | 32 |
| 9 | 2.1 | First | 31 |
| 10 | 2.3 | First | 30 |
| 11 | 2.5 | First | 29 |
| 12 | 2.8 | First | 28 |
| 13 | 3 | Second | 27 |
| 14 | 3.2 | Second | 26 |
| 15 | 3.5 | Second | 25 |
| 16 | 3.7 | Second | 24 |
| Weeks | ≈ Months | Trimester | Weeks to go |
|---|---|---|---|
| 17 | 3.9 | Second | 23 |
| 18 | 4.1 | Second | 22 |
| 19 | 4.4 | Second | 21 |
| 20 | 4.6 | Second | 20 |
| 21 | 4.8 | Second | 19 |
| 22 | 5.1 | Second | 18 |
| 23 | 5.3 | Second | 17 |
| 24 | 5.5 | Second | 16 |
| 25 | 5.8 | Second | 15 |
| 26 | 6 | Second | 14 |
| 27 | 6.2 | Second | 13 |
| 28 | 6.4 | Third | 12 |
| 29 | 6.7 | Third | 11 |
| Weeks | ≈ Months | Trimester | Weeks to go |
|---|---|---|---|
| 30 | 6.9 | Third | 10 |
| 31 | 7.1 | Third | 9 |
| 32 | 7.4 | Third | 8 |
| 33 | 7.6 | Third | 7 |
| 34 | 7.8 | Third | 6 |
| 35 | 8.1 | Third | 5 |
| 36 | 8.3 | Third | 4 |
| 37 | 8.5 | Third | 3 |
| 38 | 8.7 | Third | 2 |
| 39 | 9 | Third | 1 |
| 40 | 9.2 | Third | 0 |
How many months is 20 weeks pregnant?
About 4.6 months — you're in your fifth month, at the halfway point of a 40-week pregnancy, in the second trimester.
How many weeks is 6 months pregnant?
About 26 weeks. Six full months is roughly week 26, late in the second trimester.
When does the third trimester start?
At week 28. The second trimester runs from week 13 to week 27, and the third from week 28 until birth — both the NHS and ACOG place the start of the third trimester at 28 weeks.
What if my cycle isn't 28 days?
A longer or shorter cycle shifts ovulation, so we adjust the due date by the difference from a 28-day cycle.
Is the due date exact?
No. It's a best estimate — most babies are born within two weeks either side of it. Always follow your healthcare provider's dating.