Pregnancy Weeks to Months: Easy Conversion Chart
Convert pregnancy weeks to months with a simple chart. See exactly how many months 12, 20, or 28 weeks is — and which trimester you're in.
"I'm 12 weeks pregnant — so how many months is that?" If you've found yourself counting on your fingers at 2 AM, you're in good company. Converting pregnancy weeks to months is one of the most common questions expecting parents ask, and the math is genuinely confusing: months aren't exactly four weeks long, pregnancy is counted from before you even conceived, and 40 weeks somehow adds up to more than nine months.
This guide gives you the simple answer, a full conversion chart, and a clear explanation of why the numbers work the way they do — so the next time a relative asks "how far along are you?", you'll have the answer in whichever format they want.
The quick answer: how the conversion works
Pregnancy lasts about 40 weeks, counted from the first day of your last menstrual period. Because an average month is about 4.3 weeks long (not 4), those 40 weeks work out to roughly 9 calendar months — even though 40 divided by 4 looks like 10.
The simplest mental shortcut: divide your week number by 4.3, or just use the chart below.
Pregnancy weeks to months chart
Here's the full conversion, including which trimester each stage falls into:
| Weeks | Month | Trimester |
|---|---|---|
| Weeks 1–4 | Month 1 | First |
| Weeks 5–8 | Month 2 | First |
| Weeks 9–13 | Month 3 | First |
| Weeks 14–17 | Month 4 | Second |
| Weeks 18–22 | Month 5 | Second |
| Weeks 23–27 | Month 6 | Second |
| Weeks 28–31 | Month 7 | Third |
| Weeks 32–35 | Month 8 | Third |
| Weeks 36–40 | Month 9 | Third |
A note on precision: you'll see slightly different versions of this chart from different sources, because months and weeks don't divide evenly. Some charts put week 13 in month 3, others in month 4. Don't worry about the edges — healthcare providers track your pregnancy in weeks precisely because months are fuzzy. The chart above follows the most common convention.
12 weeks pregnant is how many months?
Since this is the question that brings most people here: at 12 weeks pregnant, you're about 3 months along, finishing up month 3 and approaching the end of your first trimester.
Week 12 is a milestone in more ways than the calendar. By now your baby is roughly the size of a plum — about 2 inches (5–6 cm) from crown to rump — with fully formed fingers, toes, and even tiny fingernails on the way. Many parents schedule their 12-week scan around now, and it's often the point where people start sharing their news.
You can see everything happening this week — baby development, common symptoms, and your checklist — on our week 12 guide.
What trimester am I in?
Trimesters split the 40 weeks into three stages. The boundaries vary slightly between sources, but the most widely used breakdown is:
- First trimester: weeks 1–13. The foundation stage — organs form, and early symptoms like nausea and fatigue tend to peak.
- Second trimester: weeks 14–27. Often called the "honeymoon trimester" — energy usually returns, your bump shows, and you'll likely feel the first kicks between weeks 18 and 22.
- Third trimester: weeks 28–40. The home stretch — baby gains weight quickly, and your body starts preparing for birth.
So at 12 weeks you're in the first trimester, at 20 weeks you're halfway through the second, and from week 28 you've entered the third.
Why does pregnancy math feel so strange?
Three quirks make week-to-month conversion confusing for almost everyone:
You start at "2 weeks pregnant" without being pregnant. Pregnancy is dated from the first day of your last period — about two weeks before ovulation, when conception actually becomes possible. So by the time a test turns positive, you're usually already counted as 4–5 weeks along. This dating method (called gestational age) is the standard your provider uses.
Months aren't four weeks. Only February is a true four-week month. Every other month is 30 or 31 days — about 4.3 to 4.4 weeks. Multiply that small difference across nine months and you get the missing "extra" month that makes 40 weeks equal 9 months, not 10.
Your due date is an estimate, not a deadline. Only about 1 in 20 babies arrives on the due date itself. Full term is considered 39 to 40 weeks, and anywhere from 37 to 42 weeks is within the normal range. Treat the due date as the center of a window, not an appointment.
If you haven't worked out your due date yet, our free pregnancy calculator does the math for you from your last period date — no email or signup needed.
How doctors count: weeks, not months
If you tell your midwife you're "five months pregnant," the first thing they'll do is convert it back to weeks. Clinical care runs on weeks (and even days — you'll see notations like "24+3", meaning 24 weeks and 3 days) because a week is a meaningful unit of development:
- Screening tests have specific week windows — for example, the nuchal translucency scan is typically done between weeks 11 and 14.
- The anatomy scan usually happens between weeks 18 and 22.
- Glucose screening commonly happens between weeks 24 and 28.
A month is simply too coarse for any of these. That's also why tracking your pregnancy week by week — rather than month by month — keeps you in sync with what's actually happening at each appointment. Our week-by-week guides follow the same structure your provider uses.
Quick reference: the milestones people ask about most
- 8 weeks ≈ 2 months — embryo officially becomes a fetus around weeks 9–10
- 12 weeks ≈ 3 months — end of the first trimester is near; risk of miscarriage drops significantly
- 16 weeks ≈ 4 months — you may start feeling flutters soon
- 20 weeks ≈ 5 months — halfway there; anatomy scan territory
- 24 weeks ≈ 6 months — viability milestone
- 28 weeks ≈ 7 months — welcome to the third trimester
- 32 weeks ≈ 8 months — baby is practicing breathing movements
- 36 weeks ≈ 9 months — full term is just a few weeks away
- 40 weeks — due date
Curious how big your baby is at each of these points? Our baby size by week guide compares every week to a fruit — from poppy seed to watermelon.
Converting the other way: months to weeks
Sometimes you need the reverse — a relative says "six months pregnant" and you want to know the week. Multiply the month by 4.3, or use these anchors: 3 months is about week 13, 5 months is about week 22, 7 months is about week 31, and "9 months pregnant" spans roughly weeks 36 to 40. And if someone asks how many weeks 9 months is, the honest answer is: pregnancy's version of 9 months is 40 weeks — that built-in extra is exactly why the weeks system exists.
The bottom line
Divide your weeks by 4.3 and you'll land on your month; check the chart above and you'll get your trimester too. But don't be surprised that everyone — your provider, your apps, this site — keeps talking in weeks. It's the unit pregnancy actually runs on, and once you start thinking in weeks, the whole timeline gets easier to follow.
This content is for informational purposes only and does not replace professional medical advice. Always consult your healthcare provider.
