How Many Weeks Is a Pregnancy?

A full-term pregnancy is 40 weeks, or 280 days, counted from the first day of your last period. That is about 9 months and 1 week. Because counting starts around two weeks before conception, the baby itself develops for about 38 weeks. Most babies are born between 37 and 42 weeks.

Why pregnancy is counted from your last period

Few people know the exact day they conceived, but most know when their last period started. So doctors and midwives date pregnancy from the first day of the last menstrual period (LMP). In a typical 28-day cycle, ovulation happens around day 14, which means you are already considered “two weeks pregnant” on the day you conceive.

This is called gestational age. It is the age used on scan reports, in antenatal notes and in pregnancy apps. Find yours with the due date calculator.

Pregnancy weeks to months

Months have different lengths, so weeks do not divide neatly into months. Using the average month of 30.4 days:

Weeks pregnant	Approximate months	Trimester
4 weeks	0.9 months	First
8 weeks	1.8 months	First
13 weeks	3.0 months	First (ends at 13 weeks 6 days)
20 weeks	4.6 months	Second
24 weeks	5.5 months	Second
28 weeks	6.4 months	Third (starts)
32 weeks	7.4 months	Third
36 weeks	8.3 months	Third
40 weeks	9.2 months	Third (due date)

Weekly counting is much more precise, which is why healthcare professionals use it.

The three trimesters

• First trimester (weeks 0–13): the baby’s organs form. In the UK, a dating scan is offered between 10 and 14 weeks.
• Second trimester (weeks 14–27): many people feel more energetic. The anomaly scan is usually between 18 and 20 weeks 6 days, and you start to feel movements.
• Third trimester (week 28 to birth): rapid growth, more frequent antenatal checks and preparation for labour.

What does 40 weeks pregnant mean?

40 weeks pregnant is your estimated due date. It is a best guess, not a deadline. The American College of Obstetricians and Gynecologists defines term like this:

Term	Weeks
Preterm	Before 37 weeks 0 days
Early term	37 weeks 0 days to 38 weeks 6 days
Full term	39 weeks 0 days to 40 weeks 6 days
Late term	41 weeks 0 days to 41 weeks 6 days
Post-term	42 weeks 0 days and beyond

Relatively few babies are born on their due date. If labour has not started by around 41 weeks, your midwife or doctor will discuss options such as a membrane sweep or induction, because risks rise slightly after 42 weeks.

How long is pregnancy from conception?

About 38 weeks, or 266 days. That is why the due date can be calculated from a known conception date by adding 266 days, and from an IVF embryo transfer by adding 261 days (day-5 embryo) or 263 days (day-3 embryo).

Will my due date change?

Your due date is often adjusted after the first dating ultrasound, which measures the baby and is more accurate than period dates, especially if your cycles are irregular or you are unsure of your last period. After that, it usually stays the same even if later scans show the baby measuring bigger or smaller.

Frequently asked questions

Is pregnancy 9 or 10 months?

About 9 months and 1 week. Forty weeks is 280 days, which is a little over nine calendar months. The “10 months” idea comes from counting 4-week months.

How many weeks is full term?

Full term is 39 weeks 0 days to 40 weeks 6 days. Babies born from 37 weeks are considered early term.

When does the second trimester start?

At 14 weeks 0 days. The third trimester starts at 28 weeks.

How many weeks pregnant am I?

Count the weeks since the first day of your last period, or enter that date in a due date calculator to see your weeks and days today.

Sources

• NHS: Your pregnancy and baby guide
• ACOG Committee Opinion 579: Definition of term pregnancy
• ACOG Committee Opinion 700: Methods for estimating the due date
• NHS: Ultrasound scans in pregnancy