How is the week of pregnancy calculated? LMP, ultrasound and the logic of 40 weeks

Counting the weeks of pregnancy can feel confusing at first. When your doctor says 6 weeks, you might think conception happened 6 weeks ago, but it is a bit different. This article explains the logic in plain language.

Why are weeks counted from the last period?

In medicine, the week of pregnancy is counted from the first day of your last menstrual period (LMP). Most people cannot know the exact day of conception, but they usually remember their last period. So week 1 is a time when you are not yet pregnant and your body is starting a new cycle.

In a regular 28-day cycle, conception generally happens about two weeks after the LMP. So actual development lasts around 38 weeks from conception, while the 40 weeks doctors count are based on the LMP. The two numbers do not conflict; they simply start at different points.

How is the due date found?

The common method is to add 280 days (40 weeks) to the LMP. As a rule of thumb, count back 3 months from the LMP and add 7 days. For example, if the LMP is 10 January, the due date is about 17 October.

This date is an estimate. Only a small share of babies arrive on that exact day; most come between weeks 37 and 42. If your cycles are irregular or clearly differ from 28 days, the LMP-based calculation can be off.

When does ultrasound come into play?

An early ultrasound can estimate the week by measuring your baby from head to rump. This measurement is quite reliable in the first trimester. According to ACOG's approach, if the gap between the LMP and ultrasound dates passes a certain limit, the doctor may change the due date to match the ultrasound. The limits depend on the week of pregnancy, and your doctor makes the call.

• If you know your LMP well and it matches the scan, the LMP is usually used.
• If the LMP is unknown or irregular, the ultrasound is the basis.
• Later scans track growth; they are not used to change the date.

If the week on your ultrasound report differs from the week in your app, go by the date your doctor has set.

How do weeks convert to months?

At 40 weeks, pregnancy is a little longer than nine months, because a month averages about 4.3 weeks. A rough conversion used in practice:

• Weeks 1–4: month 1
• Weeks 5–8: month 2
• Weeks 9–13: month 3
• Weeks 14–17: month 4
• Weeks 18–22: month 5
• Weeks 23–27: month 6
• Weeks 28–31: month 7
• Weeks 32–35: month 8
• Weeks 36–40: month 9

Trimester limits are usually given like this: the first trimester lasts until the end of week 13, the second until the end of week 27, and from week 28 until birth is the third. Sources may differ by a week or so.

What can you do?

• Note down your LMP; your doctor will ask for it at the first visit.
• If you track your cycle, show your records to your doctor.
• Enter your LMP or the due date set by your doctor in the app; weekly tracking follows from that.
• Remember that the calculation is an estimate; differences of a few days rarely matter.

When to contact your doctor

• If you have a missed period and a positive test and have not yet seen a doctor
• If you are unsure of your LMP or your cycles are very irregular, to pin down the date
• If you have any bleeding, severe belly pain or dizziness; in an emergency call 112

This article is for information only; your doctor's or midwife's advice for your own situation comes first.