To find out how many weeks pregnant you are, count the days since the first day of your last menstrual period (LMP), then divide by 7. The whole number is your completed weeks and the remainder is the extra days — so 69 days since your LMP means you are 9 weeks and 6 days pregnant, in your 10th week.
That surprises a lot of people. Pregnancy is counted from your last period, not from conception, which usually happened about two weeks later — but it is how every doctor, midwife and due date calculator counts it, so learning the method once means every appointment and scan will make sense from here on.
How Pregnancy Weeks Are Counted
Doctors date a pregnancy from the first day of your last menstrual period. This is called the LMP method, and it assumes a standard 28-day cycle with ovulation on day 14. Your due date is then set at 40 weeks — 280 days — after that first day.
Why start before conception? Because the first day of your period is a date most people can remember, while the exact day of conception is almost never known. A fixed, knowable starting point keeps you, your doctor and your ultrasound technician on the same page.
The formula is simple:
- Days pregnant = today's date minus the first day of your LMP
- Weeks pregnant = days pregnant divided by 7
- Due date = first day of your LMP plus 280 days
Try the free Pregnancy Due Date Calculator — enter your LMP or your due date and it returns your current week, your trimester and your estimated due date in seconds.
Worked Example 1: From Your Last Period
Say the first day of your last period was 1 September 2026 and today is 9 November 2026.
- September: 29 days remaining after the 1st (30 days in the month, minus day 1)
- October: 31 days
- November: 9 days so far
- Total: 29 + 31 + 9 = 69 days
- 69 ÷ 7 = 9 remainder 6, so you are 9 weeks and 6 days pregnant
You are in your 10th week of pregnancy and your first trimester. Your due date is 1 September 2026 plus 280 days, which lands on 8 June 2027.
Worked Example 2: Naegele's Rule
Doctors often use a shortcut called Naegele's rule instead of adding 280 days: take the first day of your LMP, add 7 days, subtract 3 months, then add 1 year.
Take an LMP of 1 January 2026:
- Add 7 days: 8 January 2026
- Subtract 3 months: 8 October 2025
- Add 1 year: 8 October 2026
That matches the 280-day method exactly — 1 January plus 280 days is also 8 October 2026. Naegele's rule is just a faster way of doing the same sum, and it is the method most clinics use when they write your due date on your notes.
Worked Example 3: From a Due Date
Suppose your scan gave you a due date of 22 May 2027 and you want to know how far along you are on 9 November 2026.
- Count backwards from the due date: 22 May 2027 minus 280 days gives an LMP of 15 August 2026
- Days from 15 August to 9 November: 16 days left in August, plus 30 in September, 31 in October and 9 in November = 86 days
- 86 ÷ 7 = 12 remainder 2, so you are 12 weeks and 2 days pregnant
You have just crossed into your second trimester's doorstep — week 13 begins in five days.
Week by Week: What Each Stage Means
Pregnancy is divided into three trimesters, and knowing which one you are in tells you roughly what to expect at appointments and what is developing.
| Weeks | Trimester | What is happening |
|---|---|---|
| 1–4 | First | Conception and implantation; the pregnancy test turns positive around week 4 |
| 5–8 | First | The embryo's heart starts beating; morning sickness often begins |
| 9–12 | First | All major organs have formed; the first dating scan usually happens here |
| 13–16 | Second | Energy often returns; the baby's sex may be visible on a scan |
| 17–20 | Second | First kicks are often felt; the 20-week anatomy scan |
| 21–24 | Second | The baby can hear sounds; viability milestones begin around week 24 |
| 25–28 | Third | Rapid growth; glucose screening is usually offered around week 28 |
| 29–32 | Third | The baby practices breathing movements; kicks get stronger |
| 33–36 | Third | The baby usually turns head-down; check-ups become more frequent |
| 37–40 | Third | Full term from week 37; the baby is ready to be born |
One wording trap to avoid: being "9 weeks and 6 days pregnant" means you are in your 10th week. The completed weeks are 9, but you are living through week 10 — so if an app or a doctor says "week 10", they mean the same thing.
Why Only About 5% of Babies Arrive on Their Due Date
A due date is an estimate, not an appointment. Only about 5 in 100 babies are born on their exact due date, and most arrive within two weeks either side of it. A first baby is slightly more likely to arrive late, while a second or third baby is slightly more likely to arrive a little early.
The 40-week figure is an average, not a deadline. A pregnancy is considered full term from 37 weeks, and overdue only after 42 weeks. So if your due date is 8 June and your baby arrives on 30 May, that is not early in any worrying sense — it is a normal, full-term birth. Try the Pregnancy Due Date Calculator and treat the result as the middle of a four-week window, not a single day to circle in red.
When the LMP Method Gets It Wrong
The LMP method assumes a regular 28-day cycle with ovulation on day 14. Real cycles are often longer, shorter or irregular, and the method can be off by a week or more when:
- Your cycles are longer than 35 days or shorter than 21 days
- You conceived while breastfeeding, or shortly after stopping birth control
- You are not sure of the date of your last period
- You had bleeding early in pregnancy that looked like a period but was not
In these cases the ultrasound dating scan is more reliable, especially in the first trimester. Before about 14 weeks, the baby's size (the crown-rump length) tracks gestational age very closely — usually within 3 to 5 days. Later scans are less precise because babies grow at different rates, so if your scan date and your LMP date disagree by more than a week, your care team will normally go with the scan.
What to Do With Your Number
Once you know your week, a few practical things fall into place. Book your first prenatal appointment if you have not already — most clinics like to see you by week 8 to 10. Start taking a daily folic acid supplement if you are not already, since it matters most in the earliest weeks. Many people wait until after the 12-week scan to tell others, when the risk of miscarriage drops sharply.
Keep a simple note of your week alongside your due date and trimester — a phone note is enough — so that at every appointment you can answer "how far along are you?" without doing the maths again. And remember the number changes daily: you are 9 weeks and 0 days on Monday and 9 weeks and 6 days on Sunday.
This article is general information only, not medical advice. If your dates do not add up, your cycles are irregular, or you have any worries about your pregnancy, talk to your doctor or midwife — they can date the pregnancy precisely with a scan.
Frequently Asked Questions
How do I calculate how many weeks pregnant I am?
Count the days since the first day of your last menstrual period and divide by 7. The whole number is your completed weeks and the remainder is the extra days — for example, 69 days means you are 9 weeks and 6 days pregnant.
Why is pregnancy counted from my last period instead of conception?
Because the first day of your last period is a date you can usually remember or look up, while the exact day of conception is almost never known. Counting from your last period gives everyone a fixed, shared starting point, and it adds about two weeks before conception actually happened.
What is Naegele's rule for calculating a due date?
Take the first day of your last period, add 7 days, subtract 3 months and add 1 year. For a last period starting 1 January 2026, that gives a due date of 8 October 2026 — the same result as adding 280 days.
Is my due date the day my baby will be born?
Almost certainly not. Only about 5 in 100 babies arrive on their exact due date, and most are born within two weeks either side of it. Treat your due date as the middle of a four-week window, with full term running from 37 to 42 weeks.
Try it yourself: use our free Pregnancy Due Date Calculator to get your number in seconds — no sign-up needed.