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Pregnancy Week Calculator Guide
This tells you how many weeks and days pregnant you are, which trimester you're in, and how long is left — from your last period or your due date.
Gestational age = days since the first day of your last period, expressed as weeks + days.
Due date = last period + 280 days (40 weeks).
90 days since your last period means you are 12 weeks and 6 days — written 12w6d.
Reading "12w6d"
Pregnancy is counted in completed weeks plus days, not rounded. At 12w6d you are in your thirteenth week, not your twelfth — you've completed twelve. This is why the app says one thing and your friend says another; they're counting differently.
The Trimesters
| Trimester | Weeks |
|---|---|
| First | 0w0d – 13w6d |
| Second | 14w0d – 27w6d |
| Third | 28w0d – birth |
The boundaries vary slightly between sources — some put the second trimester at 13 weeks. It makes no clinical difference; nothing changes about you on a particular Tuesday.
Why You're "Pregnant" Before You Conceived
Gestational age counts from your last period, roughly two weeks before ovulation. So for the first fortnight of your "pregnancy" you were not pregnant at all. It's a convention built on the date you can observe rather than the one you can't. Embryonic age — the actual age of the embryo — is about two weeks behind, and it's what fertility clinics use.
Term Isn't a Single Day
A due date is a midpoint, not a deadline. Only about 4% of babies arrive on it. The medically useful bands are:
Early term 37w0d–38w6d · Full term 39w0d–40w6d · Late term 41w0d–41w6d · Post term 42w0d+
Anything from 37 to 42 weeks is a normal-length pregnancy. Treating the due date as an appointment is the fastest route to a miserable final fortnight.
If Your Scan Disagrees
Trust the scan, generally. First-trimester ultrasound is more accurate than LMP dating, particularly with irregular cycles or an uncertain period date. Once your provider sets a dating due date, that becomes the official one and everything is calculated from it. Use the ultrasound due date calculator if you have a CRL measurement, and pregnancy weight gain calculator for the IOM ranges.
This is an estimate for general information, not medical advice. Every pregnancy and every cycle is different, and only your doctor or midwife can give you dates and guidance for your situation. Nothing here replaces professional care.
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