Pregnancy Calculator
Weeks pregnant are counted from the first day of the last period, not from conception. If that day was January 1, 2026, on June 18 you would be 24 weeks 0 days pregnant, in the second trimester (weeks 14 to 27), and the estimated due date is October 8, 2026.
How to use
- Choose what you know: the first day of your last period, the date of conception, or a due date from your doctor.
- Enter the date. With a last period date you can also set your usual cycle length; leave it at 28 if you are not sure.
- Read how many weeks and days pregnant you are today, your trimester and the estimated due date.
- Scroll the timeline for the dates of key weeks, from the first missed period to full term. The current stage is highlighted.
How it works
Pregnancy is dated from the first day of the last menstrual period, not from conception, which is why it lasts 40 weeks. The due date is that day plus 280 days (Naegele's rule). A cycle longer or shorter than 28 days shifts ovulation, so the date moves by the same number of days.
From a conception date the tool counts back 14 days to find the equivalent last period start. From a due date it counts back 280 days. After that all three starting points use the same calculation as the Due Date Calculator, so they give the same weeks, trimester and due date for the same pregnancy.
Trimesters follow the usual medical grouping: the first runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks until birth. Term is defined by the American College of Obstetricians and Gynecologists as early term from 37 weeks, full term from 39 weeks, late term from 41 weeks and post-term from 42 weeks.
The timeline dates are the typical timing for each item counted from the same start day. Your own care schedule can differ.
Weeks to months and trimesters
| Stage | Weeks | Starts at |
|---|---|---|
| First trimester | 0 to 13 weeks 6 days | Last period day |
| Second trimester | 14 to 27 weeks 6 days | Week 14 |
| Third trimester | 28 weeks to birth | Week 28 |
| Early term | 37 weeks 0 days to 38 weeks 6 days | Week 37 |
| Full term | 39 weeks 0 days to 40 weeks 6 days | Week 39 |
| Late term | 41 weeks 0 days to 41 weeks 6 days | Week 41 |
| Post-term | 42 weeks and beyond | Week 42 |
Method and sources
- Due date method: Naegele's rule with cycle-length adjustment, as in ACOG Committee Opinion 700, Methods for Estimating the Due Date (2017). Ultrasound dating can replace it.
- Term definitions: ACOG Committee Opinion 579, Definition of Term Pregnancy (2013).
- Typical timing of screening and vaccination items follows ACOG patient guidance and CDC guidance (Tdap between 27 and 36 weeks). Local guidelines vary.
- This tool is for general information and is not medical advice. See a doctor or midwife for prenatal care.
Sources: ACOG CO 700 · ACOG CO 579Last reviewed 2026-10-13
FAQ
How many weeks pregnant am I if I conceived on a certain date?
Add 2 weeks. A baby conceived 10 weeks ago is counted as 12 weeks pregnant, because the count starts from the last period, about 2 weeks before conception.
Why are the three start options giving the same answer?
They describe the same pregnancy. The last period start, conception (14 days later) and the due date (280 days later) are converted to one timeline.
Is the due date exact?
No. Only a small share of babies arrive on the estimated date. Most are born between 37 and 42 weeks. If an early ultrasound disagrees with the last period date by more than a week or so, your care provider will usually use the ultrasound date instead.
What do the timeline items mean for me?
They show typical timing in a pregnancy with no complications: for example the anatomy scan is usually done between 18 and 22 weeks. Your doctor or midwife decides what you need and when.
Can I date the pregnancy from an IVF transfer or an ultrasound?
This page starts from a last period, conception or due date. To get a due date from an IVF embryo transfer or from an ultrasound scan, use the Due Date Calculator, then enter that due date here for the week-by-week view.
Is my data sent anywhere?
No. Everything is calculated in your browser.