First Trimester: How Many Weeks It Lasts and What Happens

When does the first trimester start and end?

The first trimester starts on the first day of your last period and ends after week 12 in NHS, Irish and Australian guidance, or at 13 weeks and 6 days in the US. Because it is counted from your period rather than from conception, the American College of Obstetricians and Gynecologists (ACOG) notes that "an extra 2 weeks is counted at the beginning of your pregnancy when you are not actually pregnant."

• NHS (England): the week-by-week guide files everything up to week 12 under the first trimester and starts the second at week 13.
• Health Service Executive, HSE (Ireland): "The first trimester is from 0 to 12 weeks."
• Pregnancy, Birth and Baby (Australia): from the first day of your last period through to week 12.
• ACOG (US): from the first day of your last period to 13 weeks and 6 days.

So it lasts 12 weeks by the NHS, HSE and Australian count and 14 weeks by ACOG's. Pregnancy trimesters by week sets all three trimesters side by side.

What develops in the first trimester?

ACOG describes the first trimester as "the time when fertilization and major organ development occurs." For 8 weeks after fertilisation the developing baby is called an embryo; from 9 weeks after fertilisation it is called a fetus.

ACOG's guide to fetal growth lists, for weeks 1 to 8, the brain and spine beginning to form, cardiac tissue starting to develop and the lungs starting to form the tubes that will carry air. For weeks 9 to 12 it lists eyelids forming but staying closed, genitals beginning to form, the kidneys beginning to make urine and fingernails forming. For size and detail at the end of the trimester, see 12 weeks pregnant.

What are the common first trimester symptoms?

Nausea, tiredness, tender breasts and needing to wee more often are among the changes national health services describe. Pregnancy, Birth and Baby reminds you that every pregnancy is different.

• Nausea and vomiting. The NHS says morning sickness is very common in early pregnancy, can happen at any time of the day or night, and usually clears up by weeks 16 to 20. The HSE says it affects 8 out of 10 pregnant women and usually starts between 4 and 7 weeks. See what helps with morning sickness.
• Tiredness. The HSE says your baby "is growing at a rapid rate" and that this "can be tiring work for your body." See extreme tiredness in the first trimester.
• Breast changes. Pregnancy, Birth and Baby says your breasts may become tender, larger and heavier.
• Needing to wee more. Your growing uterus presses on your bladder. The HSE adds that pain when you pee or blood in your urine can mean a urine infection, which needs your GP or midwife.
• Appetite and mood. You may have food cravings, food aversions or no change in appetite at all, and hormone changes may make you moody or irritable, says Pregnancy, Birth and Baby.

Appointments and scans in the first trimester

Antenatal care starts now, and the first scan usually comes right at the end of the trimester.

Screening is your choice. The 12-week scan page explains what the dating scan measures and what happens if yours falls after 14 weeks.

Miscarriage in the first trimester

The HSE acknowledges that pregnancy loss can be common in the first trimester. ACOG calls a loss before 13 completed weeks an early pregnancy loss and says it happens in about 10 of every 100 known pregnancies.

ACOG says about half of early miscarriages happen when the embryo does not develop properly, often because of an abnormal number of chromosomes. It is direct about blame: "In almost every case, miscarriage is not a woman's fault." Working, exercising, stress, arguments, having sex, past use of the contraceptive pill and morning sickness do not cause miscarriage.

Definitions differ by country: the NHS counts a loss before 24 weeks as a miscarriage, and Pregnancy, Birth and Baby a loss before 20 weeks. Both the NHS and ACOG say most women who have a miscarriage go on to have a successful pregnancy. What happens during a miscarriage covers the practical side, and bleeding in early pregnancy explains what a threatened miscarriage means.

When to call your midwife or doctor

The NHS says to get urgent help if you are pregnant and have light vaginal bleeding or spotting (with no tummy pain, or only mild pain that does not stop you doing daily tasks), discharge that is unusual for you, or fluid leaking from your vagina. Call your maternity unit if you have the number. Before 20 weeks an early pregnancy unit may be able to help, and if you have neither, call NHS 111.

The NHS says to call 999 if you have vaginal bleeding and severe tummy pain, pain in your shoulder, feel sick, faint or dizzy, or the bleeding is heavy (soaking a period pad soon after putting it on). Our page on ectopic pregnancy covers the warning signs of a pregnancy growing outside the womb.

For sickness, the NHS says to call your midwife, GP or 111 if you are vomiting and cannot keep food or fluids down for 24 hours, have very dark urine or have not had a pee in more than 8 hours, feel very weak, dizzy or faint when standing, have tummy pain or a high temperature, vomit blood, or have lost weight. Severe sickness can be hyperemesis gravidarum, which the NHS says may need specialist treatment, sometimes in hospital.

Frequently asked questions

How many weeks is the first trimester?

Twelve weeks by the NHS, HSE and Pregnancy, Birth and Baby definitions, which all end it at week 12. ACOG in the US counts it to 13 weeks and 6 days, which makes 14 weeks. Both counts start on the first day of your last period, so the first two weeks come before conception.

When does the first trimester end?

At the end of week 12 by the NHS, HSE and Pregnancy, Birth and Baby definitions, so the second trimester begins at week 13. In the US, ACOG ends it at 13 weeks and 6 days and starts the second trimester at 14 weeks.

Is it normal to lose your appetite in the first trimester?

Yes. Pregnancy, Birth and Baby lists food cravings, food aversions and no change at all as normal first-trimester patterns. If nausea is the problem, the NHS suggests small, frequent meals of plain foods. It says to call your midwife, GP or 111 if you cannot keep food or fluids down for 24 hours or are losing weight.

When do first trimester symptoms ease?

The NHS says morning sickness usually clears up by weeks 16 to 20; the HSE says it usually starts between 4 and 7 weeks and ends around 16 to 20 weeks. Tiredness is common in the early months. The HSE says to talk to your GP or midwife if sickness is severe, especially if you cannot keep fluids down.