Second trimester: when it starts, tests and development

The second trimester is when many of the scans, tests and vaccinations of pregnancy are offered, and the US and the UK schedule several of them differently. If something worries you at any point, do not wait for the next appointment: the box above shows the emergency and urgent numbers for where you are, and get help now lists the symptoms that need checking straight away.

When does the second trimester start?

The 2nd trimester starts at week 13 or at 14 weeks, depending on the source. The NHS week-by-week guide opens its second trimester section at week 13 (NHS, "Week-by-week guide to pregnancy", 2023), and the Office on Women's Health defines the second trimester as week 13 to week 28 (Office on Women's Health, "Stages of pregnancy", 2025). ACOG defines the first trimester as running from the first day of the last period to 13 weeks and 6 days, and the second as 14 weeks 0 days to 27 weeks 6 days (ACOG, "How Your Fetus Grows During Pregnancy", 2024). Cleveland Clinic, a US hospital, puts it at around week 13 through the end of week 27 (Cleveland Clinic, "Second trimester", 2024).

So when is the second trimester, in one line? Under ACOG's lines all of week 13 is still in the first trimester, while the NHS and the Office on Women's Health count week 13 as the first week of the second. When does the 2nd trimester start for you? At whichever line your clinic uses; the difference changes no appointment, because scans and tests are booked by week. The full comparison of all three trimesters is on the hub for pregnancy trimesters, and the weeks before this one are covered in the guide to the first trimester.

Second trimester weeks, 13 to 27

This site's week pages follow the NHS grouping, so the second trimester weeks run from 13 to 27, with week 28 as the week where the sources disagree. Each page describes that week as the NHS, ACOG and the Office on Women's Health describe it.

13 weeks pregnant, 14 weeks pregnant, 15 weeks pregnant, 16 weeks pregnant, 17 weeks pregnant, 18 weeks pregnant, 19 weeks pregnant, 20 weeks pregnant, 21 weeks pregnant, 22 weeks pregnant, 23 weeks pregnant, 24 weeks pregnant, 25 weeks pregnant, 26 weeks pregnant, 27 weeks pregnant, and the boundary week, 28 weeks pregnant.

Counted in months, the 2nd trimester weeks cover roughly the fourth to the seventh month. The guide to pregnancy weeks to months gives the arithmetic, labeled as ours, since the medical sources count only in weeks.

How the baby develops in the second trimester

ACOG calls the second trimester the time of rapid growth and development (ACOG, 2024). The Office on Women's Health gives three markers (Office on Women's Health, "Stages of pregnancy", 2025):

• At 16 weeks: muscle tissue and bone continue to form a more complete skeleton, the skin begins to form, the baby makes sucking motions, and the baby is about 4 to 5 inches long and weighs almost 3 ounces.
• At 20 weeks: the baby is more active and you might feel slight fluttering; eyebrows, eyelashes and nails have formed; the baby can hear and swallow; and the baby is about 6 inches long and weighs about 9 ounces. The Office on Women's Health calls this halfway through the pregnancy.
• At 24 weeks: bone marrow begins to make blood cells, taste buds form, footprints and fingerprints have formed, the lungs are formed but do not work, the baby sleeps and wakes regularly, and the baby is about 12 inches long and weighs about 1½ pounds.

These are typical figures from one source. Every baby grows at its own rate, and a scan measures yours against the charts your clinic uses.

Scans and tests usually offered

The second trimester holds three of the best-known checks of pregnancy. Their timing and who is offered them differ between the UK and the US.

The mid-pregnancy scan

NICE guideline NG201 recommends that a scan be offered between 18 weeks 0 days and 20 weeks 6 days (NICE, "Antenatal care (NG201)", 2021); the NHS calls it the anomaly scan or 20-week scan. In the US, the Office on Women's Health says it is not uncommon to have a standard ultrasound between 18 and 20 weeks to look for signs of problems with the baby's organs and body systems (Office on Women's Health, "Prenatal care and tests", undated). It also describes a maternal serum screen, a blood test at 15 to 20 weeks that detects a higher chance of certain conditions. Screening is offered, and it is your choice whether to have it.

The glucose test in pregnancy

The CDC says gestational diabetes usually develops around the 24th week and that you will probably be tested between 24 and 28 weeks, earlier if you are at higher risk (CDC, "Gestational Diabetes", 2024). In the UK, NICE recommends checking for risk factors at the booking appointment and offering a 75 g 2-hour glucose tolerance test at 24 to 28 weeks to those with one (NICE, "Diabetes in pregnancy: management from preconception to the postnatal period (NG3)", 2015). Who is offered which test, and how each one works, is set out in the guide to the glucose test in pregnancy, and the week the testing window opens is covered on 24 weeks pregnant.

Whooping cough vaccination

The NHS recommends whooping cough vaccination for all pregnant women and says it can be given from week 16 to week 32, usually around week 20 (NHS, "Whooping cough vaccination in pregnancy", 2023). The CDC recommends the Tdap vaccine during the 27th through 36th week of each pregnancy (CDC, "Tdap Vaccination for Pregnant Women", 2025). The windows overlap only from week 27, so ask your midwife or doctor which applies to you.

Second trimester symptoms, as the Office on Women's Health lists them

The Office on Women's Health says most women find the second trimester easier than the first, and that symptoms such as nausea and fatigue may be going away while other changes become more noticeable, including feeling the baby move before the trimester is over. It lists these second trimester symptoms (Office on Women's Health, "Stages of pregnancy", 2025):

• body aches, such as back, abdomen, groin or thigh pain
• stretch marks on the abdomen, breasts, thighs or buttocks
• darkening of the skin around the nipples, a line on the skin from the belly button to the pubic hairline, and patches of darker skin on the face
• numb or tingling hands, called carpal tunnel syndrome
• itching on the abdomen, palms and soles of the feet
• swelling of the ankles, fingers and face

The same page gives two instructions to act on, and they are repeated here as it gives them. Call your doctor if you have nausea, loss of appetite, vomiting, jaundice or fatigue combined with itching, which can be signs of a serious liver problem. If you notice any sudden or extreme swelling, or gain a lot of weight very quickly, call your doctor right away, because it could be a sign of preeclampsia (Office on Women's Health, "Stages of pregnancy", 2025). This page lists what the source lists and cannot say what any symptom means for you. The box above shows who to call.

Weight gain is part of this stage too. The pregnancy weight gain calculator works out a range from your weight before pregnancy.

When the second trimester ends

ACOG ends the second trimester at 27 weeks and 6 days, the NHS guide at the end of week 27, and the Office on Women's Health at the end of week 28 (ACOG, 2024; NHS, "Week-by-week guide to pregnancy", 2023; Office on Women's Health, 2025). ACOG and the NHS therefore agree that the next stage starts at week 28, and the Office on Women's Health starts it a week later. What comes next is covered in the guide to the third trimester.

This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.