Pregnancy week by week, from 4 to 42 weeks
One page per week, built on NHS, ACOG, CDC and NICE guidance, so you can read only what matters for the week you are in.
Updated October 13, 2026 · sources checked October 13, 2026
In short
Pregnancy is counted from the first day of your last period, so week 4 is roughly when a test turns positive and week 40 is your estimated due date. Each page below covers one week: your baby's length (NHS), what you may feel, when to call for help, the tests offered in the US and the UK, and a short checklist.
How the weeks are counted
ACOG explains that pregnancy is dated from the first day of your last menstrual period, about two weeks before conception, which is why a full pregnancy is about 40 weeks. That date gives your estimated due date, which a dating scan may adjust. After IVF, ACOG says the due date comes from the embryo's age and the transfer date. Not sure where you are? The pregnancy week calculator and the due date calculator work it out in seconds, on your phone, without sending anything anywhere.
Lengths below come from the NHS week-by-week guide. The NHS measures from head to bottom until about 20 weeks, then from head to heel, so the jump at week 20 is a change of method, not a growth spurt.
First trimester: weeks 4 to 13
The weeks when the most happens and the least shows. Tiredness and nausea are common; the dating scan and screening choices come at the end. More in our first trimester guide.
| Week | Your baby (NHS) | This week in a line |
|---|---|---|
| 4 weeks | About 2 mm | Positive test, folic acid, first call to a midwife or doctor |
| 5 weeks | About 2 mm | Brain, spinal cord and heart are forming |
| 6 weeks | About 6 mm | Morning sickness, arm and leg buds |
| 7 weeks | About 10 mm | More blood volume, when to share the news |
| 8 weeks | About 16 mm | Booking appointment window opens (UK) |
| 9 weeks | About 22 mm | Pregnancy hormone at its peak |
| 10 weeks | About 30 mm | Bloating, screening choices (NIPT from 10 weeks in the US) |
| 11 weeks | About 41 mm | Placenta takes over, dating scan |
| 12 weeks | About 5.4 cm | End of the first trimester in sight, telling work |
| 13 weeks | About 7.4 cm | Gums, infections to avoid, pelvic floor |
Second trimester: weeks 14 to 27
Often the easier stretch: more energy, first movements, the 20-week scan and, in the US, the glucose test. More in our second trimester guide.
| Week | Your baby (NHS) | This week in a line |
|---|---|---|
| 14 weeks | About 8.5 cm | Energy returns, colostrum may appear |
| 15 weeks | About 10.1 cm | Balance, discharge, baby starts hearing |
| 16 weeks | About 11.6 cm | 16-week check (UK), constipation |
| 17 weeks | About 12 cm | First flutters for some, stretch marks |
| 18 weeks | About 14.2 cm | Anomaly scan window opens |
| 19 weeks | About 15.3 cm | Looser joints, sleep tips |
| 20 weeks | About 25.6 cm, head to heel | Halfway, 20-week scan, MATB1 (UK) |
| 21 weeks | About 26.7 cm | Rapid growth, whooping cough vaccine (UK) |
| 22 weeks | About 27.8 cm | Lungs practising, piles and stretch marks |
| 23 weeks | About 28.9 cm | Rib ache, telling your employer soon |
| 24 weeks | About 30 cm | Viability milestone, glucose test window (24 to 28) |
| 25 weeks | About 34.6 cm | Employer deadline (UK), swelling to mention |
| 26 weeks | About 35.6 cm | Eyes open, leg cramps |
| 27 weeks | About 36.6 cm | Last week of the second trimester, Tdap window opens (US) |
Third trimester: weeks 28 to 42
Growth, preparation, and the weeks when knowing the warning signs matters most. More in our third trimester guide.
| Week | Your baby (NHS) | This week in a line |
|---|---|---|
| 28 weeks | About 37.6 cm | Sleep on your side, RSV vaccine (UK), 28-week bloods |
| 29 weeks | About 38.6 cm | Know your baby's movement pattern |
| 30 weeks | About 39.9 cm | Vivid dreams, first baby clothes |
| 31 weeks | About 41.1 cm | Braxton Hicks, feeding plans |
| 32 weeks | About 42.4 cm | Head-down or not yet, RSV vaccine window (US) |
| 33 weeks | About 43.7 cm | Pack your bag, numbers in your phone |
| 34 weeks | About 45 cm | Lightening, leave dates |
| 35 weeks | About 46.2 cm | Pain relief options |
| 36 weeks | About 47.4 cm | Position check, GBS swab (US) |
| 37 weeks | About 48.6 cm | Early term, signs of labour |
| 38 weeks | About 49.8 cm | 38-week check, induction talk |
| 39 weeks | About 50.7 cm | Full term, warning signs, membrane sweep |
| 40 weeks | About 51.2 cm | Due date week, real contractions |
| 41 weeks | About 3 to 4 kg on average | Late term, induction offered |
| 42 weeks | Mature and ready | Postterm, monitoring or induction |
The key dates at a glance
| Timing | United States | United Kingdom (England) |
|---|---|---|
| First weeks | Call to book a first visit once your test is positive (Cleveland Clinic); blood type, urine and infection tests early on (ACOG) | Refer yourself to maternity services; booking appointment at 8 to 12 weeks (NHS) |
| 10 to 14 weeks | Cell-free DNA screening from 10 weeks; first-trimester screening at 10 to 13 weeks (ACOG) | Dating scan and optional combined screening (NHS) |
| 18 to 22 weeks | Standard ultrasound at 18 to 22 weeks (ACOG) | 20-week screening scan at 18 to 21 weeks (NHS) |
| 16 to 32 weeks | Whooping cough vaccine, usually around 20 weeks (NHS) | |
| 24 to 28 weeks | Glucose screening (ACOG) | Glucose tolerance test if you have risk factors (NHS) |
| 27 to 36 weeks | Tdap vaccine, earlier in the window preferred (CDC) | |
| From 28 weeks | RSV vaccine, every pregnancy (NHS) | |
| 32 to 36 weeks | RSV vaccine from September to January, or an antibody for your baby (CDC) | |
| 36 to 38 weeks | Group B strep swab (ACOG) | Not routinely tested |
| 41 weeks | Testing, and induction may be recommended (ACOG) | Induction offered (NICE, NHS) |
Our guide to prenatal appointments explains each visit in detail.
Help is always one call away
At any week: call your midwife, maternity unit or OB-GYN for bleeding, fluid leaking, severe or persistent tummy pain, a severe headache, vision problems, sudden swelling, a high temperature, or (from the second half of pregnancy) a change in your baby's movements. UK: NHS 111 if you cannot reach your team, 999 in an emergency. US: 911 in an emergency.
Questions parents ask
How many weeks pregnant am I?
Count from the first day of your last period, not from conception. ACOG explains that this adds about two weeks at the start, when you are not yet pregnant. Our pregnancy week calculator does the count for you; a dating scan may then adjust it.
How do I know how many weeks pregnant I am if my periods are irregular?
When the date of your last period is uncertain, ACOG says an early ultrasound can estimate gestational age by measuring the embryo from crown to rump. In England, the dating scan is offered at around 10 to 14 weeks.
How many weeks pregnant am I after IVF?
ACOG says that after IVF the due date is set from the age of the embryo and the date of the transfer. Your fertility clinic will give you your dates, and those take priority over a calculator based on periods.
Which weeks are in each trimester?
ACOG counts the first trimester up to 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks. The NHS week-by-week guide places week 13 in the second trimester, so you may see both.
Sources
- NHS — Week-by-week guide to pregnancy checked October 13, 2026
- ACOG — How Your Fetus Grows During Pregnancy checked October 13, 2026
- Cleveland Clinic — Fetal development, week-by-week stages of pregnancy checked October 13, 2026
- NHS — Antenatal (pregnancy) care and appointments checked October 13, 2026
- ACOG — Prenatal Genetic Screening Tests checked October 13, 2026
- ACOG — Routine Tests During Pregnancy checked October 13, 2026
- CDC — Vaccine recommendations before, during and after pregnancy checked October 13, 2026
- NICE NG207 — Inducing labour, recommendations 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.