How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by 7. The whole number is how many weeks pregnant you are, and the remainder is the extra days. For example, if your last period started 59 days ago, 59 ÷ 7 = 8 remainder 3, so you are 8 weeks and 3 days pregnant. This is how doctors and midwives count, as described in ACOG's guidance on estimating the due date, and it's what the calculator above does, with an adjustment if your cycles are usually longer or shorter than 28 days.
Weeks and days: what “8w3d” means
Pregnancy is tracked in completed weeks plus days. “8w3d” or “8+3” on a scan report means 8 full weeks and 3 days, which is 3 days into the ninth week. So when an app says you're “in week 9” and your notes say 8 weeks, both are describing the same day.
A pregnancy is counted as 40 weeks (280 days) from the last period to the due date. Most babies arrive in the weeks around that date rather than on it; ACOG treats 39 weeks 0 days to 40 weeks 6 days as full term.
Weeks pregnant from your last period vs from conception
Week 1 starts on the first day of your last period, about two weeks before you conceive. In a 28-day cycle ovulation happens around day 14, so the pregnancy count is roughly two weeks ahead of the embryo's actual age. If you conceived 6 weeks ago, you are about 8 weeks pregnant.
The same offset applies to IVF. On the day of a 5-day blastocyst transfer you count as 2 weeks and 5 days pregnant, and on the day of a 3-day transfer, 2 weeks and 3 days. Choose the date you know in the calculator and it applies the right offset. To work forward to the birth instead, use the due date calculator or the due date calculator by conception date.
How many months pregnant is that?
Months don't divide evenly into weeks, so there's no official conversion. Many pregnancy guides group the 40 weeks into nine months like this:
| Month | Weeks pregnant | Trimester |
|---|---|---|
| 1st month | Weeks 1–4 | first |
| 2nd month | Weeks 5–8 | first |
| 3rd month | Weeks 9–13 | first, then second from week 13 |
| 4th month | Weeks 14–17 | second |
| 5th month | Weeks 18–22 | second |
| 6th month | Weeks 23–27 | second, then third from week 27 |
| 7th month | Weeks 28–31 | third |
| 8th month | Weeks 32–35 | third |
| 9th month | Weeks 36–40 | third |
Each week page also shows the month it falls in, for example 20 weeks pregnant is in the 5th month. To convert the other way, from months to weeks, use the pregnancy weeks to months chart and converter.
When an ultrasound changes how many weeks you are
Dating from a last period assumes you ovulated about two weeks after it, which isn't always true. That's why ACOG recommends confirming the dates with an ultrasound in the first trimester, when measuring the embryo from head to bottom (the crown-rump length) is the most accurate way to estimate gestational age. If the scan and your period disagree by more than about 5 to 7 days, depending on how far along you are, your provider will usually change your due date to match the scan.
Once that happens, the scan date is the one to use. Choose “Due date” in the calculator and enter the date your provider gave you, and the weeks and days will match your notes.
Pregnancy week by week
Once you know how far along you are, each week's guide covers your baby's size and development, common symptoms, and what to do that week.
Frequently asked questions
Sources and medical references
The dating method on this page follows ACOG's guidance on estimating the due date, with the NHS and Mayo Clinic explanations of how pregnancy weeks are counted.