Most people hear "40 weeks" and picture nine months. But months are uneven — 30 or 31 days hiding in every stretch — which is why your midwife never talks in months. She talks in weeks. Once you learn the one counting rule, you can figure out your week anywhere, no app required.
The single rule: count from your period
Pregnancy is dated from the first day of your last menstrual period (LMP). Not from the day you conceived — roughly two weeks after — but from that first bleeding day. Add 40 weeks, and you have your estimated due date.
That date is called your estimated due date for a reason. Babies follow their own schedules, with most arriving between weeks 37 and 41. Think of the 40-week mark as the middle of a generous window, not a deadline.
Why the first two weeks "don't count"
Here's the part that confuses everyone: in week 1, you are not technically pregnant. Your body is still lining up ovulation, which lands around day 14 of a typical cycle. Only after that do the egg and sperm meet. But because medical dating counts from your period, those early days are included.
The reward for this quirk is consistency. A scan, a due-date estimate and a schedule of checkups can all be anchored to one date you remember — your last period.
Two steps to your week
- Find your start. Note the first day of your last period.
- Count forward. Each seven days adds one week. The +days (like "12+3") simply mean three days into week 12.
If your last period began on January 1 and today is the start of April, roughly 13 weeks have passed — you're entering week 14. A pregnancy week calculator does this instantly and also shows your trimester and due date.
Week, trimester and due date in one table
| Stage | Weeks | What's happening |
|---|---|---|
| First trimester | 1–13 | Embryo forms; morning sickness often peaks; NT scan at 11–13 weeks. |
| Second trimester | 14–27 | Growth spurt, first feels like flutters; anatomy scan at 18–22 weeks. |
| Third trimester | 28–40 | Rapid weight gain; glucose test; weekly visits from week 36 or so. |
When your week drifts from the estimate
Don't panic if a scan gives a slightly different age than your period math. Early scans measure the embryo itself, which can shift the date by a few days — sometimes more if your cycle was irregular. Your care team simply updates your "official" due date from the scan.
The key takeaway: weeks are the language of pregnancy, and one date starts the count. Master that, and you'll never be surprised by your week number again.