How many weeks pregnant am I? Week and trimester calculator
Work out your week and trimester from your due date or last period, with the movement guidance that fits each stage of pregnancy.
Runs entirely in your browser. Nothing you enter is sent anywhere.
If your due date changed after a scan, use the newest one.
This tool gives general wellness information, not medical advice. If something feels wrong, talk to your doctor or midwife.
Quick take: Pregnancy is counted from the first day of your last period, and the due date sits 40 weeks, or 280 days, after it. Enter either date to see how many weeks and days pregnant you are today, your trimester, the days left until your due date, and the movement guidance for that stage.
How it works
Knowing your week of pregnancy helps you follow your care, read about what's happening, and choose movement that fits. The calculator gives you the week, the trimester and the guidance for that stage. Here is how each part works.
How are the weeks of pregnancy counted?
By convention, the estimated due date is 280 days after the first day of the last period. That rule assumes a regular 28-day cycle with ovulation on day 14, so it can't account for longer cycles, irregular cycles, or a date that's hard to remember.[1]
This is why the date from a scan takes priority. Measuring the embryo by ultrasound in the first trimester, up to 13 weeks and 6 days, is the most accurate way to set or confirm how far along a pregnancy is.[1] If your due date changed after a scan, choose "My due date" and enter the newer date.
The science: Why period dates and scan dates can differ
Only about half of women accurately recall the first day of their last period, according to the studies summarised by the American College of Obstetricians and Gynecologists. In one study, 40% of women randomly assigned to a first-trimester scan had their due date adjusted, because the scan and the period date were more than 5 days apart.[1]
Before 14 weeks, dating by the embryo's crown-to-rump length is accurate to within about 5 to 7 days.[1]
When does each trimester start?
| Stage | Weeks | What changes for movement |
|---|---|---|
| First trimester | 0 to 13 | The basics for all weeks: talk-test effort, no breath-holding, no high-impact, high-balance or contact moves |
| Second trimester, early | 14 to 15 | Same as the first trimester |
| Second trimester, from week 16 | 16 to 27 | Also avoid long stretches lying flat on your back or front, and twisting while holding weights |
| Third trimester | 28 onwards | Also keep balance work steady and supported; standing, seated and side-lying positions tend to feel best |
How should movement change as pregnancy goes on?
The 2019 Canadian guideline recommends that all women without contraindications stay physically active throughout pregnancy.[2] The American College of Obstetricians and Gynecologists likewise encourages pregnant women without complications to continue or start safe physical activity.[3] Both expect some changes to routines as pregnancy progresses.
A few safety basics apply at every stage. The Canadian guideline advises avoiding activities with physical contact or a danger of falling, and notes that changes in body mechanics as pregnancy progresses can make falls more likely.[2] It also advises women who feel light-headed when straining while holding their breath to avoid the breath-hold.[2] That's why the guidance here always includes breathing out on the effort.
Lying flat on your back is where advice varies. The NHS suggests avoiding activity that involves lying flat on your back for more than a few minutes in later pregnancy, because it can make you feel dizzy.[5] The Canadian guideline advises changing position if you feel light-headed, nauseous or unwell on your back.[2] A systematic review found there isn't enough evidence to say whether exercising on your back is safe or should be avoided.[4] With that uncertainty, Eggsports takes the cautious route and starts this guidance at week 16.
You'll find a fuller check of effort in the pregnancy exercise intensity guide, and if you only know the date of your last period and your cycle isn't 28 days, the due date calculator can adjust for that.
When should you stop and call your doctor or midwife?
Stop and call your doctor or midwife if you notice
- Vaginal bleeding
- Fluid leaking from the vagina
- Regular painful contractions
- Dizziness or feeling faint
- Shortness of breath before you start
- Chest pain
- Headache
- Muscle weakness affecting balance
- Calf pain or swelling
If it feels urgent, call your local emergency number.
None of these signs is a reason for guilt, and stopping is always the right call. Rest is a valid choice on any day, and your own doctor or midwife is the best guide to your pregnancy.
Frequently asked questions
If I'm 24 weeks and 3 days, am I in week 24 or week 25?
Both phrases are used for the same moment. You've completed 24 weeks and 3 days, so you're in your 25th week. Midwives and doctors often write it as 24+3, and this calculator counts the same way, in completed weeks plus days.
Why is my scan date different from the date my period gives?
The period-based count assumes a 28-day cycle with ovulation on day 14, which doesn't fit everyone. An ultrasound in the first trimester measures the embryo directly and is the most accurate way to date a pregnancy. If a scan changed your due date, enter that date.
Does this work if my cycles are irregular?
The last-period option assumes a regular 28-day cycle, so it's less reliable when cycles vary. If you've had a dating scan, choose the due date option and enter the date from your scan. If not, the due date calculator lets you enter your usual cycle length.
Is anything I enter saved or sent?
No. The calculation happens in your browser, and the dates you enter are never stored, sent to us, or added to the page address. Closing the page clears them.
Sources
- American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, Society for Maternal-Fetal Medicine (2017). Methods for Estimating the Due Date. Committee Opinion No. 700. Obstetrics & Gynecology. doi:10.1097/AOG.0000000000002046
- Mottola MF, Davenport MH, Ruchat S-M, Davies GA, Poitras VJ, Gray CE, et al. (2018). 2019 Canadian guideline for physical activity throughout pregnancy. British Journal of Sports Medicine. doi:10.1136/bjsports-2018-100056
- American College of Obstetricians and Gynecologists (2020). Physical Activity and Exercise During Pregnancy and the Postpartum Period. ACOG Committee Opinion No. 804. Obstetrics & Gynecology. doi:10.1097/AOG.0000000000003772
- Mottola MF, Nagpal TS, Bgeginski R, Davenport MH, Poitras VJ, Gray CE, et al. (2019). Is supine exercise associated with adverse maternal and fetal outcomes? A systematic review. British Journal of Sports Medicine. doi:10.1136/bjsports-2018-099919
- NHS (2026). Exercise in pregnancy. nhs.uk. www.nhs.uk/pregnancy/keeping-well/exercise/