What does weeks to months mean in pregnancy?
People ask “how far along” in months. Doctors answer in weeks. This chart maps medical weeks 1–40 onto 9 months so you can translate either way without doing the maths in your head.
Free Pregnancy Tool
Pregnancy Weeks to Months
Enter a week to see the month and trimester, or scan the full conversion chart.
Use the clinical week - the number a midwife would say, from 1 to 40.
Don't know the week? Use our Pregnancy Week Calculator to find out.
Enter a week between 1 and 40.
Weeks to months chart
| Month | Weeks | Trimester |
|---|---|---|
| Month 1 | Weeks 1–4 | First trimester |
| Month 2 | Weeks 5–8 | First trimester |
| Month 3 | Weeks 9–13 | First trimester |
| Month 4 | Weeks 14–17 | Second trimester |
| Month 5 | Weeks 18–22 | Second trimester |
| Month 6 | Weeks 23–27 | Second trimester |
| Month 7 | Weeks 28–31 | Third trimester |
| Month 8 | Weeks 32–35 | Third trimester |
| Month 9 | Weeks 36–40 | Third trimester |
Why the months are uneven
Forty weeks does not split into nine equal chunks. This chart keeps months 1–3, 4–6, and 7–9 aligned with the three trimesters, which is the conversion partners actually need when someone asks “what month is she in?”
Weeks are still the number that matters
Scans, milestones, and “how far along” in clinic are all week-based. Months are a translation. Once you know the week, you already know the month - and you stop mixing the two up.
FAQ
Frequently asked questions
How pregnancy weeks map to months, and why the months are not equal.
How many months is 20 weeks pregnant?
Week 20 is month 5 of pregnancy, in the second trimester. It is also the halfway point of a 40-week pregnancy.
Why isn't pregnancy exactly nine months?
A full-term pregnancy is 40 weeks, or 280 days, counted from the last period. Nine calendar months is closer to 274 days, and 40 weeks is closer to 10 lunar months of 4 weeks. That is why clinicians use weeks, not months.
How many weeks are in each month of pregnancy?
The months are uneven because 40 weeks does not divide cleanly into 9. Months 1 and 2 are 4 weeks each, month 3 is 5 weeks (to finish the first trimester), and later months alternate between 4 and 5 weeks so months 4–6 and 7–9 line up with the second and third trimesters.
Is 12 weeks 3 months pregnant?
Yes. Week 12 falls in month 3, at the end of the first trimester. Week 13 is still month 3; month 4 starts at week 14.
More tools
Pregnancy Week Calculator
Enter your due date to instantly see your current week and day, trimester, how far along you are, and what your baby is approximately the size of.
Use tool →Baby Size by Week
See what the baby is approximately the size of at every week from 4 to 40 — no due date required.
Use tool →More from the blog
Why Pregnancy Is Measured in Weeks, Not Months
Most partners assume pregnancy lasts nine calendar months. Here is why doctors use weeks instead, and why 'five months pregnant' is harder to interpret than it sounds.
Read article →The Simple Pregnancy Timeline Every Partner Should Know
A clear breakdown of the 40-week pregnancy timeline, from trimesters to key milestones, so partners always know where things stand.
Read article →Built for dads & partners
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