Your pregnancy, trimester by trimester

One clear map of the 40 weeks, built on NHS, ACOG and CDC guidance, so you can stop searching and start enjoying.

Updated October 13, 2026 · sources checked October 13, 2026

In short

Pregnancy lasts about 40 weeks, counted from the first day of your last period, and is split into three trimesters: up to 13 weeks and 6 days, 14 to 27 weeks and 6 days, then 28 weeks to birth. Each brings its own scans and tests. Severe headache, bleeding, fluid leaking or fewer baby movements always deserve a call straight away.

The three trimesters at a glance

ACOG defines the trimesters by completed weeks, counted from the first day of your last period. Knowing where you are tells you what your body is doing, which tests are coming and what you can prepare.

Trimester	Weeks	What is happening for your baby	Key appointments
First	Up to 13 weeks + 6 days	Brain, spine and heart tissue begin to form; organs develop	First visit, blood tests, dating scan (NHS: 10 to 14 weeks)
Second	14 weeks to 27 weeks + 6 days	Rapid growth; hearing develops; first movements felt	Anatomy scan (NHS: 18 to 21 weeks), glucose test at 24 to 28 weeks
Third	28 weeks to birth	Weight gain; lungs, brain and nervous system mature	More frequent checks, group B strep test at 36 to 38 weeks (US)

Not sure how far along you are? Our due date calculator and pregnancy week calculator do the arithmetic in seconds, privately in your browser.

Trimester by trimester

First trimester: weeks 1 to 13 covers the early tiredness and nausea, folic acid, your first appointment and the dating scan.
Second trimester: weeks 14 to 27 covers the energy comeback, the anatomy scan, first kicks and the glucose test.
Third trimester: weeks 28 to 40 covers side-sleeping, baby movements, the last checks and getting ready for birth.

Feeling well: food, movement and sleep

Most questions in pregnancy come back to three things: what you eat, how you move and how you rest.

Topic	The short version	Read more
Food safety	Avoid foods linked to listeria and high mercury, cook meat and eggs well	Foods to avoid in pregnancy
Drinks	Caffeine under 200 mg a day; no alcohol	Caffeine and alcohol limits
Blood sugar	Screening at 24 to 28 weeks for most people	Gestational diabetes
Movement	150 minutes of moderate activity a week if your pregnancy is uncomplicated	Exercise during pregnancy and prenatal yoga
Sleep	On your side from 28 weeks	Sleeping positions

Common discomforts and what helps

Morning sickness: small, frequent, plain meals and when it becomes serious.
Heartburn: eating and sleeping habits that ease it.
Back pain and sciatica: posture, movement and support.
Pregnancy pillow guide, maternity belt and compression socks: what each one is for and how to choose.

Is this normal? When to call

Tiredness, mild aches, nausea and a bit of anxiety are part of most pregnancies. The NHS and the CDC list the symptoms that should never wait for your next appointment.

Symptom	Why it matters
Severe headache, or one that will not go away	Can be a sign of pre-eclampsia
Sudden vision changes, blurring or flashing lights	Can be a sign of pre-eclampsia
Sudden swelling of the face, hands or feet	Pre-eclampsia or a blood clot (DVT)
Vaginal bleeding, or fluid leaking	Needs checking at any stage
Your baby moving less, or a change in the pattern	Call your maternity unit immediately, day or night
Fever (NHS: 38°C or above; CDC: 100.4°F or higher)	Can be a sign of infection
Chest pain, trouble breathing, fainting	Needs urgent care
Thoughts of harming yourself or your baby	Help is available now

In the UK, call 999 for a life-threatening emergency and NHS 111 if you are not sure what to do. In the US, call 911 in an emergency. Otherwise, call your midwife, maternity unit or OB-GYN, whatever the hour.

Every family, every path

Solo, with a husband, a wife or a co-parent, pregnant naturally or after IVF: every guide here is written for you. If you conceived through IVF, your clinic will give you your dates, and the trimesters work the same way from there.

Printable checklist: this week

Check how many weeks pregnant I am
Save my midwife, maternity unit or OB-GYN number in my phone
Read the warning signs above once, then stop worrying about them
Pick one guide for this trimester and read it

Get organised

Pregnancy checklist by trimester: what to book, buy and decide, and when.
Prenatal appointments: the US and UK schedules side by side.
Maternity clothes sizing: how to measure and when to size up.

All pregnancy guides

Pregnancy week by week, from 4 to 42 weeksEvery week from 4 to 42: baby's length, symptoms, tests in the US and UK, and a checklist.The first trimester, weeks 1 to 13Weeks 1 to 13: symptoms, folic acid, first appointment and the dating scan.The second trimester, weeks 14 to 27Weeks 14 to 27: first movements, the anatomy scan, the glucose test and vaccines.The third trimester, weeks 28 to 40Weeks 28 to 40: side-sleeping, baby movements, last tests and getting ready for birth.Early signs of pregnancy, and when to take a testThe first symptoms, when they start, and when a home pregnancy test is most reliable.Foods to avoid in pregnancy, and the ones you can enjoyCheese, deli meat, fish, eggs, liver: what to skip, what to heat, what is fine.Caffeine and alcohol in pregnancyThe 200 mg caffeine limit in everyday drinks, and why alcohol is best avoided altogether.Gestational diabetes: test, daily life and after birthWho is screened, how the glucose test works, eating and moving with GD, and checks after birth.Exercise during pregnancy: what is safe and how much150 minutes a week: what counts, what to avoid, and the signs to stop.Prenatal yoga: a gentle way to stay strong and calmWhat the research shows, the adjustments that matter, and how to choose a class.Morning sickness: what helps, and when to callWhen it starts and ends, what helps, and the signs of hyperemesis gravidarum.Heartburn in pregnancy: causes, relief and when to callWhy it happens, eating and sleeping habits that help, remedies to ask about.Sleeping positions in pregnancy, trimester by trimesterWhich side, when to stop sleeping on your back, and how to build a comfortable pillow nest.Back pain and sciatica in pregnancy: what helps and when to callLow back pain, pelvic girdle pain and sciatica: how to tell them apart, daily fixes and warning signs.How to choose a pregnancy pillow that fits the way you sleepU, C, J, wedge or straight: what each shape does, which fill to pick and how to keep it clean.Maternity belts and belly bands: which one, and how to wear itBelly band, support belt, pelvic belt: what each does, how to fit it and when a physio should choose it.Compression socks in pregnancy: what they are for and how to chooseSwelling, varicose veins and travel over 4 hours: when compression comes up, sizing and warning signs.Maternity clothes sizing: buy your usual size, at the right timeHow maternity sizes work, how to measure and what to buy in each trimester without wasting money.Your pregnancy checklist, trimester by trimesterAppointments, scans, vaccines and practical to-dos for each trimester, with the weeks they happen.Your prenatal appointments, week by week (US and UK)Visit frequency in the US, the NHS antenatal schedule, scans, tests and vaccines at each stage.

Questions parents ask

How are the weeks of pregnancy counted?

From the first day of your last menstrual period, not from conception. ACOG explains that this adds about two weeks at the start, which is why a full-term pregnancy is described as about 40 weeks.

How likely am I to give birth on my due date?

Not very. ACOG notes that only about 1 in 20 women give birth on their due date. The date is still useful because it sets the timing of your scans and tests.

Who should I call if something feels wrong?

Your midwife, maternity unit or OB-GYN office first. In the UK, call NHS 111 if you are unsure and 999 in an emergency; in the US, call 911 in an emergency.

Sources

ACOG — How Your Fetus Grows During Pregnancy checked October 13, 2026
ACOG — Routine Tests During Pregnancy checked October 13, 2026
NHS — Pregnancy symptoms you need to get help for checked October 13, 2026
CDC — Urgent Maternal Warning Signs (Hear Her) checked October 13, 2026
NHS — 20-week scan checked October 13, 2026
NHS — Foods to avoid in pregnancy (caffeine limit) checked October 13, 2026
ACOG — Exercise During Pregnancy checked October 13, 2026
NHS — Stillbirth (going to sleep on your side after 28 weeks) checked October 13, 2026

Mamavela explains; it does not replace your midwife, doctor or pediatrician, who know your situation. If something worries you, call them. In an emergency, call 911 (US), 999 (UK) or 112.