Count the days since the first day of your last period and divide by 7. The whole number is your completed weeks and what is left over is the days: 120 days is 17 weeks and 1 day. Doctors count from that day, and the average pregnancy lasts 280 days, or 40 weeks (ACOG).
How are pregnancy weeks counted?
Pregnancy is counted from the first day of your last period, not from the day you conceived. The average length of a pregnancy is 280 days, or 40 weeks, counted that way (ACOG, the American College of Obstetricians and Gynecologists). ACOG explains that pregnancy is assumed to start 2 weeks after that day, so the first two weeks are counted before you are actually pregnant (ACOG). The NHS (the UK’s National Health Service) says pregnancy normally lasts from 37 to 42 weeks (NHS). About 1 woman in 20 gives birth on her estimated due date (ACOG).
ACOG defines gestational age as how far along you are in your pregnancy, usually reported in weeks and days (ACOG). This page gives completed weeks and days, so 17 weeks and 1 day means you are in your 18th week. The Period Calc app shows the completed weeks, so the same day is Week 17 there. The due date calculator on this site counts the week you are in and shows Week 18 of 40.
The formula
Weeks and days = days since the first day of the last period ÷ 7 (the whole number is the weeks, what is left over is the days)
Due date = first day of the last period + 280 days
From a due date: first day of the last period = due date − 280 days
A worked example
Your last period started on and you want to know your week on .
- Days since the last period started
- 120 days
- Weeks and days
- 17 weeks and 1 day (120 ÷ 7 = 17, with 1 day left over)
- Week you are in
- the 18th (the app shows Week 17, the due date calculator Week 18 of 40)
- Trimester
- second (14 weeks to 27 weeks and 6 days)
- Due date
- ( + 280 days)
- Days left
- 160 days (280 − 120)
- In the Hijri calendar
- last period on , that day , due date
Which trimester am I in?
A trimester is one of the three parts of pregnancy. ACOG defines them in weeks and days (ACOG):
- First trimester: up to 13 weeks and 6 days
- Second trimester: 14 weeks and 0 days to 27 weeks and 6 days
- Third trimester: 28 weeks and 0 days to 40 weeks and 6 days
The calculator uses these lines, and keeps saying third trimester after 40 weeks and 6 days. Other guides draw them a little differently. The NHS week-by-week guide begins the second trimester at week 13 (NHS). It does not say how it counts a week. Week 13 starts at 12 or 13 completed weeks, both at or before ACOG’s 14 weeks and 0 days.
How many months pregnant am I?
The months on this page are plain arithmetic, not a medical rule. A calendar month is about 4.3 weeks (30.4 days), so 40 weeks (280 days) is about 9 months and a week. ACOG’s month-by-month chart counts differently: it ends at month 10, with weeks 37 to 40 (ACOG).
Each row below is the start of a completed week, for example 13 weeks and 0 days. At 13 weeks you have finished 2 months and are just short of the third.
Weeks, trimester and whole calendar months passed, rounded down (our arithmetic, not a medical rule)
| Weeks pregnant | Trimester | Months passed |
|---|---|---|
| 6 weeks | First trimester | 1 |
| 8 weeks | First trimester | 1 |
| 12 weeks | First trimester | 2 |
| 13 weeks | First trimester | 2 |
| 14 weeks | Second trimester | 3 |
| 16 weeks | Second trimester | 3 |
| 20 weeks | Second trimester | 4 |
| 24 weeks | Second trimester | 5 |
| 27 weeks | Second trimester | 6 |
| 28 weeks | Third trimester | 6 |
| 32 weeks | Third trimester | 7 |
| 36 weeks | Third trimester | 8 |
| 40 weeks | Third trimester | 9 |
| 42 weeks | Third trimester | 9 |
Why can a scan change my week?
If the date of your last period is uncertain, ACOG says an ultrasound exam may be done in the first trimester to estimate the due date (ACOG). In England the NHS offers a dating scan at around 10 to 14 weeks to work out how many weeks pregnant you are and your due date (NHS). The due date may be adjusted according to the ultrasound measurements (NHS). ACOG adds that once a due date has been selected, it does not change no matter how many more ultrasound exams you have (ACOG). If your doctor or midwife gives you a date, follow theirs.
When to see a doctor or midwife
If a test says you are pregnant, tell a doctor or midwife as soon as you can. In England the NHS says it is important to tell the NHS as soon as possible, so you can get the care and information you need (NHS). If you are not sure when your last period started, speak to a midwife or GP (family doctor) (NHS).
The NHS lists symptoms to get help for in pregnancy, among them “Change in your baby's movements”, “Fluid leaking from your vagina”, “High temperature (fever)” and “Severe headache”. It says vaginal bleeding in pregnancy is not always serious, but it is important to get advice from a healthcare professional (NHS). It also says: “If you have any other symptoms or you're worried about your health or your baby's health, get help” (NHS).
More tools: the conception date calculator and the pregnancy test date calculator. Questions about the app: the Period Calc FAQ.
Estimates only, not medical advice. A doctor or midwife can tell you how far along you are.
Questions about pregnancy weeks
Why am I already 4 weeks pregnant when my period has only just been missed?
Because the count starts on the first day of your last period, not when you conceived. ACOG says the first 2 weeks are counted before you are actually pregnant (ACOG). With a 28-day cycle, your next period is due when the count reaches 4 weeks.
Is 40 weeks the same as 9 months?
I am not sure of the date of my last period. Can I still use this?
You can enter your best guess, or your due date if a doctor gave you one, and treat the result as approximate. If you do not know the date or are unsure, the NHS says to speak to a midwife or GP (family doctor). It says the 12-week scan can estimate more accurately how many weeks pregnant you are (NHS).
Which trimester am I in at 13 weeks?
The first. ACOG’s first trimester runs to 13 weeks and 6 days, and the second starts at 14 weeks and 0 days (ACOG). Other guides draw the lines a little differently, so ask your midwife which one they use. See which trimester am I in.
Why does my doctor’s week differ from the calculator’s?
How this page was made
Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the health organisations listed under Sources. They have not reviewed it, and neither has a doctor. The calculator runs the formula above in your browser. The 280-day rule and the trimester lines are the ones the Period Calc app uses; the months table is our own arithmetic. The calculator works from averages, so it cannot know how your own body differs from them, and it cannot diagnose anything: if something worries you, see when to see a doctor.
Sources
- Pregnancy due date calculator (NHS)
- 12-week scan (NHS)
- Ultrasound scans in pregnancy (NHS)
- Week-by-week guide to pregnancy (Best Start in Life) (NHS)
- What to do when you find out you're pregnant (NHS)
- Pregnancy symptoms you need to get help for (NHS)
- When Pregnancy Goes Past Your Due Date (ACOG)
- How Your Fetus Grows During Pregnancy (ACOG)
- Changes During Pregnancy (ACOG)