Pregnancy Trimesters Broken Down by Weeks and Months

How are the pregnancy trimesters broken down by weeks?

Pregnancy is split into three trimesters, each lasting about three months or 13 weeks, counted from the first day of your last period rather than from conception. In the NHS week-by-week guide the first trimester runs to the end of week 12, the second covers weeks 13 to 27 and the third starts at week 28. The American College of Obstetricians and Gynecologists (ACOG) draws the first line a week later, at 13 weeks and 6 days.

ACOG puts a normal pregnancy at about 40 weeks from the first day of your last menstrual period. Because pregnancy is assumed to start two weeks after that date, ACOG notes that "an extra 2 weeks is counted at the beginning of your pregnancy when you are not actually pregnant." Those two weeks sit inside the first trimester.

ACOG defines trimesters simply as "the 3-month periods of time in pregnancy", which is where the months column comes from. Weeks do not divide evenly into calendar months, so for the month a particular week falls in, use the pregnancy weeks to months converter.

Why do the NHS, ACOG and other health services give different weeks?

Forty weeks does not split into three equal blocks of whole weeks, so any scheme has to round somewhere, and these health services round in different places. The disagreement is almost entirely about week 13, with one outlier at week 27.

So at 13 weeks pregnant you are in the second trimester by the NHS and HSE reckoning and still in the first by ACOG's. At 27 weeks the NHS, ACOG and HSE all say second trimester, while Australia's Pregnancy, Birth and Baby says third. From 28 weeks everyone agrees you are in the third trimester.

None of this changes your care. Your notes, scans and appointments are dated in weeks and days, not trimesters; ACOG gives the example that "36 and 3/7 weeks" means 36 weeks and 3 days.

What happens in each trimester?

ACOG gives each trimester a one-line purpose, and it is a useful way to think about the three stages.

• First trimester is "the time when fertilization and major organ development occurs". Pregnancy, Birth and Baby says you might have nausea in these weeks, and that your midwife or doctor will usually offer an ultrasound scan towards the end of the trimester (see the 12-week scan). See the full first trimester guide.
• Second trimester is "the time of rapid growth and development". The HSE says this is when you will begin to feel your baby moving, with a fetal anatomy scan at 18 to 22 weeks (see the 20-week scan) and diabetes screening between 24 and 28 weeks. See the second trimester guide.
• Third trimester is "the time when the fetus's weight increases and the organs mature so they will be ready to function after birth". The HSE says appointments become more frequent; in Australia, Pregnancy, Birth and Baby puts them at about every 4 weeks until 36 weeks, then every 2 weeks. See the third trimester guide.

For what is happening in a particular week, the week-by-week pages go into more detail than a trimester overview can.

How long is a full-term pregnancy?

About 40 weeks, or 280 days, from the first day of your last period. ACOG and the Society for Maternal-Fetal Medicine no longer treat "term" as a single five-week window; they endorse splitting it into four bands:

The reason, ACOG explains, is that the frequency of problems for newborns is lowest in uncomplicated pregnancies delivered between 39 weeks 0 days and 40 weeks 6 days. Australia's Pregnancy, Birth and Baby uses the broader definition: a baby born in weeks 37 to 42 is considered full term, and one born before week 37 is premature.

Your due date is an estimate, not an appointment. ACOG says only about 1 in 20 women give birth on their due date. The due date calculator shows how it is worked out.

How do you know which week, and trimester, you are in?

From your due date, which is worked out from the first day of your last period. ACOG explains that when that date is uncertain, an ultrasound in the first trimester measures the crown–rump length (the length from the top of the head to the bottom) and a formula converts it into weeks. If you had IVF, your due date is set from the age of the embryo and the date it was transferred.

Once your due date is fixed, every week and trimester follows from it. That is why a scan that moves your due date also moves which trimester you are in. The gestational age calculator works it out from your due date, last period or conception date.

Frequently asked questions

What trimester is 27 weeks?

The second trimester, according to the NHS week-by-week guide, ACOG in the US and the HSE in Ireland. All three start the third trimester at 28 weeks. Australia's Pregnancy, Birth and Baby is the exception: it says that when you reach week 27 you are in the third and final trimester.

What trimester is 13 weeks?

It depends whose definition you use. The NHS guide files week 13 under the second trimester, and the HSE puts the second trimester at 13 to 28 weeks. ACOG counts the first trimester up to 13 weeks and 6 days, so in the US you are still in the first. By 14 weeks, every source agrees it is the second.

How many weeks is each trimester?

About 13 weeks, according to Pregnancy, Birth and Baby. The exact split depends on the source. By ACOG's dates the first and second trimesters are each 14 weeks long and the third is 13 weeks to 40 weeks 6 days. By the NHS guide the first is 12 weeks and the second 15.

How many weeks is a pregnancy?

About 40 weeks from the first day of your last period, which ACOG notes includes two weeks before you were actually pregnant. In singleton pregnancies the average is 40 weeks, or 280 days. ACOG calls a birth between 39 weeks 0 days and 40 weeks 6 days full term.