How pregnancy weeks are counted
Doctors and midwives count a pregnancy from the first day of your last menstrual period, not from conception. That day is week 0, day 0, and a full-term pregnancy is 40 weeks (280 days) from it. Because ovulation and conception usually happen about two weeks into a cycle, you're already around 2 weeks pregnant when you conceive, and about 4 weeks pregnant when your period is late.
Weeks are written as completed weeks plus days: "12 weeks 3 days" (often shortened to 12w3d or 12+3) means 12 full weeks and 3 days. You're then in week 13 of your pregnancy.
How far along am I based on my due date?
Your due date is day 280, so you can work backwards from it. Count the days from today to your due date and subtract them from 280. The answer is how many days pregnant you are; divide by 7 for weeks.
| Days until your due date | How far along you are |
|---|---|
| 210 days | 10 weeks 0 days |
| 140 days | 20 weeks 0 days |
| 100 days | 25 weeks 5 days |
| 60 days | 31 weeks 3 days |
| 11 days | 38 weeks 3 days |
Pregnancy weeks to months
Forty weeks doesn't divide evenly into months, so pregnancy guides group the weeks into nine months like this:
| Month of pregnancy | Weeks | Trimester |
|---|---|---|
| Month 1 | Weeks 1 to 4 | First |
| Month 2 | Weeks 5 to 8 | First |
| Month 3 | Weeks 9 to 13 | First |
| Month 4 | Weeks 14 to 17 | First and second |
| Month 5 | Weeks 18 to 22 | Second |
| Month 6 | Weeks 23 to 27 | Second |
| Month 7 | Weeks 28 to 31 | Second and third |
| Month 8 | Weeks 32 to 35 | Third |
| Month 9 | Weeks 36 to 40 | Third |
Counting calendar months gives a slightly different answer: 38 weeks 3 days is month 9 of pregnancy, but only about 8.8 calendar months since your last period. The calculator shows both.
Trimesters and term
ACOG counts the first trimester through 13 weeks 6 days, the second from 14 weeks through 27 weeks 6 days, and the third from 28 weeks. A baby born from 37 weeks is early term, from 39 weeks full term, from 41 weeks late term, and from 42 weeks post-term. The due date calculator lists the date you reach each one.
How exact is "how far along"?
The count is exact; the starting point is an estimate. Dating from your last period assumes you ovulated about two weeks in, and an early ultrasound often corrects it by a few days. Even with accurate dates, birth timing varies: in a study that pinpointed ovulation with daily hormone tests, only 4% of women gave birth at exactly 280 days, and 70% within 10 days of their due date (ESHRE, 2013).