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Pregnancy Week by Week: Your Baby's Development and Body Changes (Weeks 1–40)
Pregnancy lasts about 40 weeks across three trimesters, counted from the first day of your last period.
Quick Facts
Pregnancy is an extraordinary journey of growth — from a tiny cluster of cells to a fully formed baby ready to meet you. Following it week by week helps you understand what's happening inside, know what's normal, and feel connected to your baby at every stage.
Pregnancy is counted from the first day of your last period and lasts about 40 weeks, divided into three trimesters. In the first trimester your baby grows from a poppy-seed-sized embryo to a plum-sized foetus with a beating heart; by the second trimester you may feel movements; and in the third, your baby gains weight and gets ready for birth.
This pillar guide gives you the big picture of all 40 weeks and links to detailed week and trimester pages. It's general information — your midwife or doctor is your guide for your specific pregnancy.
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Things worth knowing
Counted from your last period
Pregnancy is dated from the first day of your last period, not from conception.
Heartbeat by about week 12
A heartbeat can usually be seen on ultrasound by around 12 weeks.
First movements in the second trimester
Many feel the baby move during the second trimester; first-time mums may notice later.
The dating scan sets your dates
Done around 11–14 weeks, it confirms your due date and is used throughout pregnancy.
Folic acid and vitamin D matter early
Take folic acid until 12 weeks and vitamin D as advised to support development.
Due dates are only estimates
Just about 1 in 20 babies arrive exactly on the due date.
Everything You Need to Know (The 40-Week Overview)
Pregnancy spans three trimesters across about 40 weeks (with dates confirmed by your dating scan). Here's the big picture, plus a few development snapshots along the way.
| Trimester | Weeks | Highlights |
|---|---|---|
| First | 1–12 | Major organs form; heartbeat detectable on scan by about week 12; baby grows to roughly plum size |
| Second | 13–27 | Baby grows rapidly; you may feel movements; the 20-week anomaly scan happens |
| Third | 28–40 | Baby gains weight, lungs mature, and moves into position for birth |
| Week 4 | Embryo about 2 mm (poppy seed); amniotic sac and yolk sac present | |
| Week 6 | About 6 mm (pea); limb buds appear | |
| Week 8 | About 16 mm (raspberry); embryo becomes a foetus | |
| Week 12 | About 5.4 cm (plum); heartbeat seen on ultrasound; organs formed |
Due dates are estimates
Only about 1 in 20 babies arrive exactly on the due date; "full term" is 37–42 weeks. Your dating scan (around 12 weeks) sets the date used through pregnancy.
Symptoms / Signs (Common by Stage)
What many people experience — though everyone differs:
First trimester
- Missed period
- Nausea or "morning sickness"
- Tiredness
- Tender breasts
- Needing to wee often
- Food aversions
Second trimester
- Often more energy
- A growing bump
- First movements ("quickening")
- Some aches as the body changes
Third trimester
- More tiredness
- Breathlessness
- Heartburn
- Swelling
- Braxton-Hicks (practice) contractions
- Baby movements strong then changing as space reduces
Track while you read
Tick the symptoms that apply to you. This is a self-check, not a diagnosis — saved on this device only.
Always report
Reduced baby movements (third trimester), heavy bleeding, severe headache with vision changes or swelling (possible pre-eclampsia), severe abdominal pain, fever, or waters breaking — these need prompt medical assessment.
How Pregnancy Progresses
Pregnancy unfolds through coordinated changes:
- Fertilisation & implantation
- The embryo embeds in the womb lining.
- Placenta & cord form
- Supplying oxygen and nutrients to the baby.
- Organogenesis (first trimester)
- Major organs and structures form — which is why early health and folic acid matter.
- Growth & maturation (second / third)
- The baby grows, and the senses and lungs develop.
- Hormonal changes
- Drive both the baby's development and your body's adaptations.
Confirming & Dating Your Pregnancy
How pregnancy is confirmed and dated:
Steps in confirming and dating
- Pregnancy test — detects hCG (from the first day of a missed period)
- Booking appointment — your first midwife appointment (usually before about 10 weeks), with history and routine blood tests
- Dating scan (about 11–14 weeks) — confirms dates and the number of babies; the scan date is used throughout
- Ongoing antenatal care — regular checks; see Tests & Scans
Store scan reports and notes in your ParentVibes Medical Records so they're easy to share at appointments.
Antenatal Care Through the Weeks
Pregnancy isn't an illness, but regular antenatal care keeps you and baby healthy. A typical schedule looks like this:
- Booking (by about 10 weeks)
- History, blood tests, advice, and folic acid / vitamin D.
- About 12 weeks
- Dating scan plus combined screening (offered).
- About 20 weeks
- Anomaly scan.
- 28 weeks
- Repeat bloods, anti-D if needed, and growth checks.
- Later weeks
- More frequent checks: blood pressure, urine, growth, position, and movements.
Take care with supplements and medicines
Attend all antenatal appointments, take recommended supplements (folic acid, vitamin D) as advised, and never take medicines without checking they're pregnancy-safe — see Nutrition.
Looking After Yourself Each Week
Healthy habits across pregnancy:
Everyday habits
- Eat well and take folic acid (until 12 weeks) and vitamin D — see Nutrition
- Stay active with pregnancy-safe exercise (as advised)
- Rest and sleep — listen to your body, especially in trimesters 1 and 3
- Avoid smoking, alcohol, and unsafe foods or medicines
- Track movements in the third trimester and report changes promptly
- Mind your wellbeing and reach out for support if you need it
Weekly self-care checklist
- Supplements taken
- Balanced meals & hydration
- Gentle activity
- Rest
- Movements noted (3rd trimester)
When to See a Doctor
Contact your midwife or doctor promptly if you have:
- Reduced or changed baby movements (third trimester) — report straight away
- Vaginal bleeding or fluid leaking (possible waters breaking)
- Severe or constant headache, vision changes, or sudden swelling (possible pre-eclampsia)
- Severe abdominal pain, fever, or persistent vomiting
- Signs of early labour before 37 weeks
- Any worry about your or your baby's wellbeing
Seek urgent / same-day care
Seek urgent or same-day care for heavy bleeding, severe pain, reduced movements, or signs of pre-eclampsia. Use Ask a Doctor on ParentVibes for guidance (not emergencies), or contact your maternity unit.
Continue learning
Frequently Asked Questions
How is pregnancy counted, and how long is it?
It's counted from the first day of your last period and lasts about 40 weeks (roughly 9 months), in three trimesters.
When can I hear the heartbeat?
A heartbeat can usually be seen on ultrasound by around 12 weeks.
When will I feel my baby move?
Often during the second trimester; first-time mums may notice it a little later. Report reduced movements in later pregnancy.
Is my due date exact?
No — it's an estimate, and few babies arrive exactly on it. Full term is 37–42 weeks.
What scans will I have?
Usually a dating scan (around 12 weeks) and an anomaly scan (around 20 weeks), plus screening tests offered along the way — see Tests & Scans.
What should I avoid during pregnancy?
Smoking, alcohol, certain foods, and unsafe medicines — see Nutrition.
Is some discomfort normal?
Yes — nausea, tiredness, aches and heartburn are common, but report severe or worrying symptoms.
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Medical review
- Last reviewed
- June 2026
- Medical reviewer
- Dr. Vinika G.
- Next review due
- June 2027
- Status
- Medically reviewed by Dr. Vinika G.
References
This article is for general information and education only and is not a substitute for professional antenatal care. Every pregnancy is different. Always follow the advice of your midwife or doctor, attend antenatal appointments, and report any concerning symptoms promptly. Never take medicines without confirming they are safe in pregnancy. In a medical emergency, contact your maternity unit or local emergency services immediately. Content reviewed against guidance from the NHS.
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Medical disclaimer
This article is for educational purposes only and does not replace medical advice. If you have severe pain, heavy bleeding, missed periods, or unusual symptoms, please consult a qualified healthcare provider.