Pregnancy weeks to months: how the counting actually works

Ask how far along someone is, and a doctor will answer in weeks while friends and family usually ask in months. Converting between the two is surprisingly confusing, because pregnancy is dated from before conception and because months don't divide evenly into weeks. Here's how it works.

Pregnancy is counted from your last period

By convention, pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. Conception usually happens about two weeks later, around ovulation. So in the first two "weeks" of pregnancy, you aren't actually pregnant yet. It sounds odd, but the LMP is a date most people know, while the exact day of conception usually isn't.

That's why a full-term pregnancy is described as 40 weeks, even though the baby develops for about 38 weeks from conception.

Reading weeks and days

Providers usually describe gestational age as completed weeks plus days, written like 24w3d, meaning 24 weeks and 3 days. At 24w3d you've finished 24 weeks and are partway through your 25th week. Some apps and books label that as "week 25", which is why sources can seem to disagree by one.

Why weeks don't convert neatly to months

Forty weeks is 280 days, which is a little over nine calendar months, not exactly nine. Calendar months average about 4.3 weeks, so charts that convert weeks to months have to round. Different charts round differently, so you may see the same week assigned to different months. A commonly used breakdown:

• Month 1: weeks 1 to 4
• Month 2: weeks 5 to 8
• Month 3: weeks 9 to 13
• Month 4: weeks 14 to 17
• Month 5: weeks 18 to 22
• Month 6: weeks 23 to 27
• Month 7: weeks 28 to 31
• Month 8: weeks 32 to 35
• Month 9: weeks 36 to 40

Treat this as an approximation for conversation. Medical care always uses weeks and days because they're precise.

Trimesters

Trimesters are the other common way to divide pregnancy. The first trimester runs through 13 weeks and 6 days, the second from 14 weeks through 27 weeks and 6 days, and the third from 28 weeks until birth. Each is roughly three months long, though the third can run longer if the baby arrives after the due date.

Term definitions

Late in pregnancy, the weeks matter for specific medical definitions. According to the American College of Obstetricians and Gynecologists:

• Preterm: before 37 weeks
• Early term: 37 weeks through 38 weeks 6 days
• Full term: 39 weeks through 40 weeks 6 days
• Late term: 41 weeks through 41 weeks 6 days
• Post-term: 42 weeks and later

Find your own week

The pregnancy week calculator shows exactly how many weeks and days along you are from your last period or conception date, along with your trimester. If your provider has given you a due date from an ultrasound, you can use the due date calculator in reverse to work out the matching LMP, which keeps everything consistent with the dates your care team uses.