How many months, how many weeks? Pregnancy weeks, months and trimesters

When someone asks “How many months along are you?”, the maths can get muddled, because healthcare professionals think in weeks and the people around you think in months. This guide brings the two together to simplify your road map.

How are pregnancy weeks counted?

Pregnancy weeks are counted from the first day of your last period. Ovulation and conception usually happen around week 2, so in the first two weeks there is no baby yet, but the calendar is running. This is why your baby's true age is about two weeks less than the pregnancy week.

A full pregnancy lasts about 40 weeks, or 280 days. The due date is calculated from this and can be refined with an early ultrasound. Most babies are born a few weeks before or after that date; weeks 37 to 42 are generally accepted as the window for birth.

How do weeks translate into months?

Calendar months do not hold an equal number of weeks, so the conversion is approximate. It can shift by a week or two depending on the source; still, this translation works well most of the time:

• Month 1: weeks 1–4
• Month 2: weeks 5–8
• Month 3: weeks 9–13
• Month 4: weeks 14–17
• Month 5: weeks 18–22
• Month 6: weeks 23–26
• Month 7: weeks 27–30
• Month 8: weeks 31–35
• Month 9: weeks 36–40

A pregnancy is not exactly nine months; it runs about nine and a half calendar months, and 40 weeks can also be split into ten “months” of four weeks. So it is not surprising to see different counts in different sources.

The three trimesters

A pregnancy is usually divided into three stages:

• First trimester (weeks 1–13): your baby's basic organs take shape. Nausea, tiredness and mood changes are common.
• Second trimester (weeks 14–27): many people feel more energetic. The first baby movements are noticed in this stage.
• Third trimester (weeks 28–40): the baby gains weight quickly and has less room. Preparation for birth gets busier.

The boundaries can vary by about a week between sources; what matters is the overall flow.

Milestones to know on the calendar

The ranges below are general information; your own timing is set with your doctor or midwife:

• Weeks 4–14: 1st antenatal visit; basic tests
• Weeks 6–10: early ultrasound; seeing the heartbeat
• Weeks 11–14: nuchal translucency and combined screening
• Weeks 18–22: detailed ultrasound
• Weeks 24–28: glucose tolerance test
• Weeks 28–34: anti-D for Rh-negative pregnancies, as planned by your doctor
• Weeks 35–37: GBS screening
• Weeks 37–42: monitoring with NST if needed

For details, see the screening and check-up calendar in the app.

What you can do

• Note down the date of your last period; once entered in the app, the weeks are calculated automatically.
• Use “weeks” when you talk to your doctor; saying “I am at 30 weeks” instead of “in my seventh month” avoids confusion.
• Plan appointments by week and add them to your calendar so you do not miss screening windows.
• Do not compare details with someone else's pregnancy; weeks are standard, but every pregnancy moves at its own pace.

This article is for information only; the right schedule for you is set by the advice of your doctor or midwife.