Pregnancy Weeks to Months
Full weeks-to-months chart
| Weeks | Month | Trimester |
|---|---|---|
| 1–4 | Month 1 | 1st |
| 5–8 | Month 2 | 1st |
| 9–13 | Month 3 | 1st |
| 14–17 | Month 4 | 2nd |
| 18–22 | Month 5 | 2nd |
| 23–27 | Month 6 | 2nd |
| 28–31 | Month 7 | 3rd |
| 32–35 | Month 8 | 3rd |
| 36–40+ | Month 9 | 3rd |
Curious about exact upcoming dates instead of weeks? Try the Weeks From Today Calculator.
How this is calculated
Doctors and midwives track pregnancy in weeks because it's precise and easy to compare against milestones like scans and screening windows. But a "pregnancy month" isn't a clean four weeks — a 40-week pregnancy divided evenly into 9 months works out to roughly 4.4 weeks per month, not 4. That's why the chart above groups weeks unevenly: some months span 4 weeks, others span 5, so that 40 weeks lines up with 9 months by the end.
There's no single official standard for exactly where one "month" ends and the next begins, so you may see slightly different week-to-month charts elsewhere. The version above is one of the most common groupings used by care providers and pregnancy apps, but treat the month label as a friendly approximation — your actual week of pregnancy is always the more precise number.
Common questions and things to know
Some people count pregnancy as 10 lunar months instead of 9 calendar months — that's not a mistake, it's just a different unit. A lunar month is roughly 4 weeks, so 40 weeks divides evenly into 10 lunar months, while a calendar month averages about 4.3 weeks, giving 9 calendar months for the same pregnancy.
Due dates and "40 weeks" are themselves estimates — full term is generally considered 37 to 42 weeks, so your actual delivery month may not match your calculated one exactly. If your care provider has given you a different week count from an early ultrasound, that dating generally takes priority over a last-period calculation.
It's also worth remembering that "month 9" doesn't mean the pregnancy is nearly finished the moment it starts — it spans weeks 36 through 40, so there's still a meaningful stretch of time (and plenty of change) left within that final month. Trimesters follow a similar logic: they're a rougher three-way split of the same 40 weeks, useful for talking about broad stages of development rather than a precise weekly marker.
Sources
General pregnancy timeline and prenatal care guidance: American College of Obstetricians and Gynecologists (ACOG) · Mayo Clinic · NHS.