How many weeks pregnant am I?
If you just found out you're pregnant, the first question is almost always the same: how far along am I? The answer is counted in weeks — and the way those weeks are counted surprises almost everyone.
Pregnancy weeks start before conception
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived. Conception usually happens around two weeks after your period starts, which means by the time a test turns positive you're typically already considered 4 to 5 weeks pregnant.
This is the convention doctors and midwives use worldwide, because the LMP is a date most people actually know, while the exact day of conception usually isn't certain.
How to work out your week
- Find the first day of your last period. Count the number of full weeks from that day to today. That's your week of pregnancy.
- Know your conception date instead? Add two weeks to align with medical dating, then count from there.
- Already have a due date from your provider? Count backwards: your due date marks week 40, so 70 days before it you're at week 30, and so on.
Weeks are usually written with days too — "20 + 3" means 20 weeks and 3 days. Those extra days matter for scans, screening windows, and milestones, so it's worth tracking your progress at day-level precision rather than rounding.
Weeks, months, and trimesters
A full-term pregnancy is about 40 weeks, which is a little more than nine months — another reason weeks are the unit everyone uses. Weeks 1–13 are the first trimester, 14–27 the second, and 28 to birth the third. If you want the full breakdown, see our guide to pregnancy trimesters.
Let Lunera answer it for you — every day
Instead of counting on a calendar every time someone asks, enter one date in the Lunera pregnancy tracker — your last period, conception date, or due date — and the home screen always shows your exact week and day, like "Week 20 + 3 days", along with your trimester and a countdown to your due date.
- Add a widget and your week is on your lock screen without opening the app
- Ask Siri "What week of pregnancy am I on?" and get the answer hands-free
- A weekly notification tells you the moment a new week begins
Quick answers
Why am I 4 weeks pregnant if I just conceived?
Because pregnancy is counted from the first day of your last menstrual period, which is usually about two weeks before conception. A positive test around a missed period therefore lands at roughly 4 weeks.
How accurate is week tracking from my last period?
It's the standard method providers use, and it's accurate to within a few days for most people with regular cycles. An early ultrasound may adjust the estimate; if it does, update your tracker with the new due date.
Track your pregnancy with Lunera
Week-by-week baby development, due date countdown, widgets, and a one-tap contraction timer — free, private, and subscription-free on iPhone.
This guide is for informational and educational purposes only and is not medical advice. Lunera is not a medical device. Always consult your doctor, midwife, or healthcare provider with any questions about your pregnancy or labor.