How Many Weeks Pregnant Am I From My Last Period?
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: How many weeks pregnant am I from my last period? Count the days from the first day of your last period to today and divide by 7. The whole number is your completed weeks; the remainder is days. So 66 days is 9 weeks 3 days. Your due date is that first day plus 280 days.
This guide walks you through the count step by step, shows a weeks-to-months table and trimester dates, and explains when a scan will replace the number. For an instant answer, use our free How Many Weeks Pregnant Am I? calculator.
Step-by-step: count your weeks from your last period
- Find the first day of your last period (LMP). This is the day bleeding started, not the day it stopped. The NHS calculator uses the same date.
- Count the days from then until today. Day one is the first day of your period.
- Divide by 7. The whole number is your completed weeks.
- Note the remainder. That's the extra days. Write it as weeks+days, for example 9+3.
- Find your due date: first day of your LMP + 280 days.
- Check your trimester using the table further down.
Worked example
- First day of last period: 3 August 2026
- Today: 8 October 2026
- Days between: 66
- 66 ÷ 7 = 9, remainder 3 → 9 weeks 3 days pregnant (9+3)
- Due date: 3 August 2026 + 280 days = 10 May 2027
At 9+3 you're "9 weeks pregnant" and in your 10th week, just as a 9-month-old baby is in their 10th month. Open the 9 weeks pregnant page to see what's happening this week.
Shortcut without counting days
Count the number of full weeks from the same weekday. If your LMP was a Monday, every Monday you go up a week. Three Mondays after your LMP, you're 3+0. That's how most apps tick over to a new week.
Why it's counted from your last period
Pregnancy is dated from your LMP because it's a date most people know, and ovulation, about 2 weeks later, usually isn't. That means you're counted as about 2 weeks pregnant on the day you conceive. Our guide on why pregnancy calculators add 2 weeks explains this in full.
Weeks vs months table
Pregnancy is 40 weeks from your LMP, a little over 9 calendar months. Months don't divide evenly into weeks, so different sources round differently. This table uses roughly 4⅓ weeks per month.
| Month of pregnancy | Weeks (from LMP) | Trimester |
|---|---|---|
| Month 1 | 1–4 | First |
| Month 2 | 5–8 | First |
| Month 3 | 9–13 | First |
| Month 4 | 14–17 | Second |
| Month 5 | 18–22 | Second |
| Month 6 | 23–27 | Second |
| Month 7 | 28–31 | Third |
| Month 8 | 32–35 | Third |
| Month 9 | 36–40 | Third |
If you want the full explanation of why some people say "10 months", see is pregnancy 9 or 10 months?
Trimester boundaries
| Trimester | ACOG (US) | Common NHS wording (UK) |
|---|---|---|
| First | Week 1 to 13 weeks 6 days | Weeks 1 to 12 |
| Second | 14 weeks 0 days to 27 weeks 6 days | Weeks 13 to 27 |
| Third | 28 weeks 0 days to birth | Weeks 28 to 40 |
The US and UK draw the first line about a week apart. It doesn't change your care; it's just a label. Read more in when does the second trimester start?
Key dates by week
Once you know your week, these are worth putting in your calendar:
| Week | What usually happens |
|---|---|
| 4 | Period due; home test may be positive |
| 8–12 | First midwife or prenatal appointment (the NHS "booking" appointment) |
| 10–14 | Dating scan and first-trimester screening |
| 18–21 | Anatomy (20-week) scan |
| 24–28 | Glucose test for gestational diabetes, if offered |
| 28 | Third trimester starts; get to know your baby's movements |
| 36 | Ask when to call or come in during labor |
| 37 | Early term |
| 40 | Due date |
The full list is in our pregnancy appointment schedule.
When a scan replaces your LMP count
Your LMP count is an estimate. ACOG says a first-trimester ultrasound, up to 13 weeks 6 days, is the most accurate way to confirm how far along you are. If the scan and your LMP differ by more than about 5 to 7 days in the first trimester, your provider will use the scan date, and your week number may move. See can your due date change?.
Your LMP count is more likely to be off if:
- your cycles are irregular, or much longer or shorter than 28 days
- you're not sure of the date your last period started
- you had bleeding in early pregnancy that you thought was a period
- you got pregnant soon after stopping hormonal contraception or while breastfeeding
Track your weeks in Momi
Momi works out your due date from the first day of your last period or a known conception date and follows your pregnancy week by week. Each week shows your baby's size and development, and the home screen shows how many weeks you have to go. If your scan gives you a new date, update it and everything moves to match.
Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Related: due date calculator · how is due date calculated
Sources
- NHS, Pregnancy due date calculator
- ACOG, Committee Opinion 700: Methods for Estimating the Due Date
- ACOG, Changes During Pregnancy (infographic)
- NHS, Your antenatal appointments
Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.
FAQ
How many weeks pregnant am I from my last period?
Count the days from the first day of your last period to today, then divide by 7. The whole number is your completed weeks and the remainder is extra days. For example, 66 days is 9 weeks and 3 days, written 9+3. That's your gestational age.
Do I count from the first or last day of my period?
From the first day, the day bleeding started. Doctors, midwives and every due date calculator use the first day of your last menstrual period (LMP) as day one of pregnancy.
If I'm 9 weeks and 3 days, am I in week 9 or week 10?
You're 9 weeks pregnant and in your 10th week. Completed weeks are counted, the same way a baby who is 9 months old is in their 10th month. Apps and scan reports usually show this as 9+3 or 9w3d.
What if I don't know when my last period started?
Give your best estimate and book an appointment. Your midwife or doctor can arrange a dating scan, which measures the baby and gives an accurate gestational age, especially in the first trimester. Until then, treat any calculator result as a rough guide.
My cycle is longer than 28 days. Is the count still right?
The week count from your LMP is the standard, but with a long cycle you probably ovulated later, so your baby may measure a little behind that count on an early scan. If the difference is big enough, your provider will move your due date to match the scan.
Related guides
Track this in Momi
Momi is a free pregnancy tracker app for iPhone and Android that follows pregnancy week by week, estimates the due date, and keeps a pregnancy journal, weight log, kick counter and contraction timer in one place.
