How to calculate your pregnancy week
The simple counting rule that dates your whole 40-week journey.
Pregnancy dating
When is your due date?
Find out from your last period, ovulation date, IVF transfer or an early ultrasound — then see which week you're in, how big baby is, and what's coming next.
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Estimated due date
—Currently in your
—Trimester
—Days to go
This week
Week —
Baby's size
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Weight
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Checkups ahead
Share it
Your pregnancy week card
Dates are estimates. Your care team will set your official due date — always follow their guidance and your scan findings.
Why 40 weeks?
Every pregnancy is dated the same way
An average pregnancy lasts 280 days from the last period — about 40 weeks. Because people often don't know exactly when they ovulated, LMP dating is the standard. Ovulation, IVF and ultrasound dating simply adjust the starting point to get closer to the truth.
- Pick a start point — period, ovulation, transfer or scan.
- Enter the date and see your due date instantly.
- Watch the week flip as baby grows inside.
- Share your card with the people cheering you on.
Due date questions
Is the due date the exact birth day?
Rarely. A due date is the midpoint of a window — most babies arrive within about two weeks either side of it. Only around four in 100 are born exactly on their due date.
Why do doctors go by my last period?
It's the date most people can pin down with confidence. Counting 40 weeks from there gives a standard frame to schedule scans and check growth, even if the true conception happened later.
Can IVF give a more accurate due date?
Yes. Because the embryo transfer date is known precisely, IVF dating is the most exact method — a day-5 transfer is dated at 2 weeks 5 days and the due date is counted from there.
Why does my ultrasound age differ from my period age?
Early scans measure the embryo to estimate age, which can differ from LMP by a few days or more if your cycle was irregular. Scans in the first trimester are considered very reliable.