Pregnancy Week by Week: The Complete Guide

Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. Because ovulation usually happens about two weeks after your period starts, you're already counted as '2 weeks pregnant' at the moment you actually conceive. It feels odd, but it gives everyone — you, your midwife, your scans — a single shared starting line. A full-term pregnancy is 40 weeks from LMP, which is about 38 weeks of actual fetal development.

Your due date is an estimate, not a deadline: only about 1 in 20 babies arrive on it. Anything from 37 to 42 weeks is considered term. Your dating scan (around 12 weeks) measures the baby and may adjust your due date by a few days.

The first trimester does the most invisible work. By the end of week 12, your baby has grown from a single cell to a fully formed fetus with a beating heart, developing limbs, fingers and the beginnings of every major organ. You may not look pregnant yet, but this is the most biologically intense phase.

What's happening to your baby

• Weeks 4-6: the neural tube (brain + spinal cord) forms — this is why folic acid before and through week 12 matters so much. The heart starts to beat around week 6.
• Weeks 7-9: limbs, fingers and toes appear; facial features form.
• Weeks 10-13: all major organs are in place; the baby starts to move (though you can't feel it yet) and is about the size of a plum by week 13.

What you may feel

• Nausea and vomiting ('morning sickness') — peaks around weeks 8-10 for most people and eases by ~14 weeks.
• Deep fatigue, sore breasts, frequent urination, heightened smell and food aversions.
• Mood swings from surging hormones. All of this is normal and usually settles in the second trimester.

Most people feel dramatically better in the second trimester — nausea fades, energy returns, and the bump finally shows. It's often called the 'honeymoon' phase of pregnancy.

What's happening to your baby

• Weeks 14-18: the baby can make facial expressions, hear sounds and is covered in fine hair (lanugo).
• Weeks 18-22: the 20-week anomaly scan checks the heart, brain, spine, kidneys and limbs in detail. You can often find out the sex if you want to.
• Weeks 23-27: the baby practises breathing movements, gains fat, and would have a chance of survival with intensive care if born now.

What you may feel

• First kicks ('quickening') — usually between 16 and 24 weeks (later for first pregnancies).
• A visible bump, more appetite, and possibly round-ligament pain as your uterus stretches.
• Skin changes (a darker line down the belly, the 'pregnancy glow') and occasional heartburn.

The final trimester is about growth and preparation. Your baby gains roughly half their birth weight in these weeks, your body gets ready for labour, and appointments become more frequent.

What's happening to your baby

• Weeks 28-32: rapid brain growth; the baby opens their eyes and is increasingly responsive to light and sound.
• Weeks 33-36: the baby usually settles head-down; lungs mature; you may notice hiccups.
• Weeks 37-40: 'term'. The baby is fully developed and putting on the final layers of fat, ready to be born.

What you may feel

• Breathlessness, heartburn, swollen ankles, back ache and Braxton-Hicks ('practice') contractions.
• Trouble sleeping — sleep on your side (ideally left) from 28 weeks, as back-sleeping is linked to higher stillbirth risk.
• Increased pressure as the baby drops ('lightening') in the final weeks.

Schedules vary by country and by whether this is your first baby, but the UK NHS pattern is a useful map. Expect roughly 7-10 appointments in a first uncomplicated pregnancy.

• By 10 weeks: booking appointment — bloods, blood pressure, health history, and your folic acid + vitamin D check.
• 11-14 weeks: dating scan + (optional) combined screening for Down's, Edwards' and Patau's syndromes.
• 18-21 weeks: anomaly scan — detailed check of the baby's development.
• 16, 25, 28, 31, 34, 36, 38, 40, 41 weeks: midwife checks (blood pressure, urine, bump measurement, baby's position). Extra growth scans if needed.
• 28 weeks: glucose tolerance test if you're at higher risk of gestational diabetes; anti-D injection if you're rhesus negative.

Most pregnancies are healthy, but some symptoms need same-day assessment. Call your midwife, maternity triage or 999 if you have:

• Vaginal bleeding, or fluid leaking from the vagina (possible waters breaking).
• Reduced, slowed or stopped baby movements (from ~24 weeks).
• A severe or persistent headache, vision changes (spots/blurring), or sudden swelling of the face, hands or feet — possible signs of pre-eclampsia.
• Severe tummy pain, a high temperature, or burning when you wee.
• Persistent vomiting where you can't keep fluids down (possible hyperemesis).
• Regular, painful contractions before 37 weeks.