Doctors count pregnancy in weeks, but everyone asks in months — and because months aren’t all four weeks long, the conversion is messier than it looks. Set your week below to see which month and trimester you’re in, how far along you are, and how the full 40 weeks map onto nine months.
Why 40 weeks make 9 months
A calendar month averages about 4.35 weeks, not 4 — so 40 weeks of pregnancy is roughly 9¼ calendar months, not 10. That’s why a simple “divide by four” puts you a month ahead of where you really are. The mapping used here follows the common obstetric convention: months 1–3 cover weeks 1–13 (first trimester), months 4–6 cover weeks 14–27 (second), and months 7–9 cover weeks 28–40 (third).
Keep in mind that clinicians will always talk in weeks and days ("24 and 3") because a month is too blunt an instrument — babies change enormously within a single month. Use months for family and friends, weeks for your doctor, and this page whenever the two need translating.
Common questions
Nine-and-a-bit calendar months, or ten “lunar” 4-week months — both describe the same 40 weeks. The confusion comes from months being longer than four weeks. Counted on a real calendar, 280 days is about 9¼ months.
Because precision matters: screening tests, scans and medication windows are all timed to specific weeks, sometimes specific days. A month can contain two very different stages of development, so “weeks and days” is the clinical standard everywhere.
The usual convention: the first trimester is weeks 1–13, the second is 14–27, and the third is 28 until birth. You’ll see slightly different cut-offs in some sources — the boundaries are conventions, not biological switches.
Find your week in the table: weeks 1–4 are month 1, 5–8 month 2, 9–13 month 3, 14–17 month 4, 18–22 month 5, 23–27 month 6, 28–31 month 7, 32–35 month 8, and 36–40 month 9.
Count from the first day of your last menstrual period — not from conception. If you know your due date instead, you’re 40 weeks minus however long is left. Our due date calculator works this out for you from either date.
ℹ️Medical disclaimer. This tool is for general information only and isn’t medical advice, diagnosis or treatment. Estimates can differ from your real dates — always confirm them with your doctor or midwife.