Pregnancy is counted as 40 weeks and also described as 9 months. Those two sentences only fit if some months contain more than four weeks. Dividing weeks by 4, or by 4.345, answers a different question.
The chart below is the one the weeks-to-months calculator uses. It is a reading guide for week-by-week schedules. It is not a medical stage and it does not date the pregnancy by itself.
Chart months
| 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 | 36–42 |
Week 12 is month 3. Week 20 is month 5. Week 28 is month 7. Week 40 is month 9.
Week 13 is still month 3 on this chart, and it is also the start of the second trimester in the trimester dates used on this site. Month and trimester are separate labels.
Calendar months
40 weeks ÷ 4.345 is about 9.2 calendar months. 20 weeks is about 4.6 calendar months, while the chart calls week 20 month 5. Use the chart when a book says “month 5.” Use the decimal only when you specifically want calendar months.
The calculator prints both so the two answers stay next to each other.
Getting the week count first
The chart needs a week number. Count days since the first day of the last period and divide by 7, which is what the pregnancy week calculator does. How many weeks pregnant am I is that division with an example. If the period date is unknown, the week count has to come from an ultrasound rather than from this table.