Pregnancy Week by Week: What Changes and When

Pregnancy is counted in weeks from the first day of your last period and runs to about 40 weeks, divided into three trimesters. The first trimester is when the organs form and early symptoms are usually strongest. The second brings growth, a visible bump and the first movements you can feel. The third is mostly weight, lung maturity and getting into position for birth.

How the weeks are counted

Weeks are counted from the first day of your last menstrual period, not from conception. That is why you are already described as four weeks pregnant at about the time a test turns positive, even though the pregnancy itself is younger than that. Dating this way is a convention that makes every pregnancy comparable, and it is also why your week number is written as two numbers: 12+3 means twelve completed weeks and three days.

A dating scan in the first trimester measures the baby and may move your estimated due date by a few days in either direction. If that happens, the scan date usually becomes the one your team works from. There is more on that in how your due date is calculated.

The first trimester: weeks 4 to 13

This is the busiest stretch developmentally and often the hardest one to feel good in. The neural tube, heart, limbs and facial features all form in these weeks, and by the end of the first trimester the baby has recognisable hands, feet and a working heartbeat, at a size usually compared to a lime or a plum.

What you may feel varies enormously. Nausea, exhaustion that arrives out of nowhere, sore breasts, a sharper sense of smell, food aversions and needing to urinate more often are all common. Some people feel none of it, and that is not a warning sign. Your first antenatal appointment, often called the booking appointment, usually happens in this trimester, and the NHS describes it as the longest one because it covers your history, blood tests and what care you will be offered.

The second trimester: weeks 14 to 27

Most people find this the easiest trimester. Early sickness often eases, energy returns, and the bump becomes visible somewhere around the middle of it. The baby grows fast, starts to hear, swallows amniotic fluid and settles into sleep and wake cycles.

The landmark here is movement. Between 16 and 24 weeks you are likely to notice the first flutters, and over the following weeks they become recognisable kicks, rolls and hiccups. The anomaly scan, usually offered between 18 and 21 weeks, also falls in this trimester.

The third trimester: weeks 28 to 42

From 28 weeks the work shifts from building to finishing. The lungs mature, the baby puts on fat, and by around 36 weeks most babies have moved head-down in preparation for birth. Appointments become more frequent, and you may be offered checks on growth and position.

Physically this is the demanding trimester again: heartburn, breathlessness, back and pelvic pain, swollen feet, broken sleep and Braxton Hicks tightenings are all common. Side sleeping is advised from the third trimester onwards, which is covered in the best sleeping position in pregnancy.

A quick map of the three trimesters

Trimester	Weeks	Mostly about	Commonly offered care
First	4–13	Organ formation, early symptoms	Booking appointment, dating scan
Second	14–27	Growth, first movements, visible bump	Anomaly scan, routine checks
Third	28–42	Weight, lungs, position for birth	More frequent appointments, growth and position checks

How to use a week-by-week tracker without spiralling

Reading ahead is the most common way a tracker stops being useful. A few habits keep it helpful:

• Read your current week, and read next week if you want to plan. Reading week 34 at week 12 mostly buys you worry.
• Treat size comparisons as a rough sense of scale, not a measurement. Fruit is not a growth chart.
• Ignore anything framed as “you should be feeling” by now. Symptom lists describe a population, not you.
• Write down what you actually feel. Your own notes over three weeks are worth more to your midwife than any article.
• If a week’s content makes you anxious about something specific, that is a question for your next appointment, not a reason to search further at 2am.

When not to wait for next week

Some things are never a “mention it at the next appointment” matter. Contact your midwife, maternity unit or doctor now, at any hour, if you have any of these:

• Your baby’s movements slow down, change or stop after they were established.
• Any vaginal bleeding.
• Your waters break, or you have fluid leaking.
• A severe or persistent headache, flashing lights in your vision, or sudden swelling of your face, hands or feet.
• Sudden one-sided swelling, pain or redness in a leg.
• A fever, or feeling generally very unwell.
• Severe or constant abdominal pain.

Nobody minds being called. Being checked and sent home is the expected outcome, not a waste of anyone’s time.

Following your week in PreBaby

PreBaby counts your week and day from the due date or last period you enter, and shows one screen for the week you are actually in: what is developing, what you may be feeling, a size comparison with its own illustration, and one thing worth doing. It covers weeks 4 to 42, and the logs sit alongside it, so the symptoms you note down are in the same place as the week they belong to when you next see your midwife.

Sources

• NHS — Pregnancy
• NHS — Your antenatal appointments
• NHS — Your baby’s movements
• World Health Organization — Antenatal care recommendations

This article is general information, not medical advice. Anything that worries you is a reason to contact your own midwife or doctor.