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How pregnancy is divided

Pregnancy is counted as 40 weeks from the first day of your last menstrual period, and it is conventionally divided into three trimesters of roughly equal length:

• First trimester: weeks 1 to 13
• Second trimester: weeks 14 to 27
• Third trimester: weeks 28 to birth

The exact boundaries vary slightly between sources — some place the second trimester start at 13 weeks, others at 14. The difference is a matter of convention rather than anything biological, and a few days either way changes nothing about your pregnancy.

Why the count starts before conception

This confuses almost everyone at first. Dating from the last menstrual period means you are counted as two weeks pregnant at the moment of conception, before a pregnancy exists at all.

The reason is practical: the date of a period is something most people can identify, while the date of conception usually is not. Ovulation timing varies between people and between cycles, so a system based on it would be less reliable. The convention has simply proved the most consistent starting point available.

This is also why the “40 weeks” of pregnancy involves roughly 38 weeks of actual fetal development.

What each trimester generally involves

First trimester. The period of most rapid development, when major organs and body systems form. It is also when nausea, extreme tiredness, and tender breasts are most common. Many people feel at their worst during these weeks while showing no outward sign of pregnancy at all, which can be an isolating combination.

Second trimester. Often described as the most comfortable stretch. Early nausea commonly settles, energy frequently returns, and most people begin to show. First movements are usually felt somewhere between 16 and 25 weeks — later in a first pregnancy. The mid-pregnancy anomaly scan generally falls around 18 to 21 weeks.

Third trimester. Growth accelerates and physical discomfort tends to increase — back pain, heartburn, breathlessness, and disturbed sleep are all common. Appointments become more frequent, and preparation for birth moves to the foreground.

Terms you will hear about timing

• Preterm: before 37 weeks
• Early term: 37 weeks to 38 weeks and 6 days
• Full term: 39 weeks to 40 weeks and 6 days
• Late term: 41 weeks to 41 weeks and 6 days
• Post term: 42 weeks and beyond

Only a small proportion of babies arrive on their exact due date. Most first babies arrive somewhat after it. A due date is best understood as the centre of a range rather than an appointment.

Using this as a rough guide

Trimesters are a convenient way to talk about pregnancy, but nothing changes overnight when one ends. Symptoms do not arrive or disappear on schedule, and there is enormous variation between people and between pregnancies.

If you feel very different from what a stage is supposed to feel like, that is usually normal variation rather than a sign of a problem. Anything that worries you — bleeding, severe pain, reduced movements after they have been established, severe headache, or sudden swelling — is worth contacting your midwife or maternity unit about straight away, whatever week you are in.

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