How pregnancy weeks are counted
Pregnancy is counted from the first day of the last menstrual period, which means week 1 and week 2 technically begin before conception even happens. A '40-week pregnancy' is therefore 38 weeks of real embryonic and fetal development plus 2 weeks of counting head start. This odd-sounding convention is universal: your doctor, your hospital letter and every app on your phone all use it.
Gestational age is expressed in weeks and days — 22+3 means 22 weeks and 3 days. Our Pregnancy Week Tracker converts your due date into this exact format, and tells you which week you are in today, how far along you are as a percentage, and which trimester you are crossing.
The key habit for parents-to-be: anchor yourself to your due date (or your scan date), let the tracker do the counting, and never worry about a single-day discrepancy between apps — a few days of difference is pure arithmetic noise.
First trimester: weeks 1–13
In the first trimester, a single fertilized egg becomes a fully recognizable little human. By week 4 the embryo is a few millimeters long; by week 8 the major organs have begun to form; by week 12 the fetus has fingers, toes, eyelids and a heartbeat that ultrasound can clearly detect.
This is also the trimester of symptoms: nausea, fatigue, breast tenderness and mood swings are driven by a surge of hormones — hCG doubling early, then progesterone and estrogen rising steadily. Nausea peaks around weeks 6–9 and eases for most women by week 12–14, though 'most' still leaves many exceptions.
Practical milestones: your first prenatal visit usually lands around week 8–10, the first-trimester combined screening around weeks 11–13, and the NT ultrasound in the same window. Week 13 marks the official end of the first trimester for most care protocols.
Second trimester: weeks 14–27
The second trimester is famously the easiest stretch for most women. Energy often returns, nausea fades, and the bump becomes visibly pregnant rather than 'just full'. Around weeks 16–20, many women feel the first flutters of fetal movement — quickening — which later turn into unmistakable kicks.
Development races ahead: by week 20, the fetus is around 25 cm and covered in fine lanugo hair; by week 24, its lungs are developing surfactant, the substance that lets them inflate after birth, and a baby born at this point has — with intensive care — a real chance of survival.
Screening milestones: the anomaly scan ('anatomy scan') happens around weeks 18–22 and is the big structural check; glucose screening for gestational diabetes usually lands between weeks 24 and 28. Our week tracker highlights these windows so nothing sneaks past on a busy calendar.
Third trimester: weeks 28–40
The third trimester is the homestretch — and the most physically demanding. The fetus gains weight rapidly, from roughly 1 kg at week 28 to about 3–3.5 kg at term. Braxton Hicks contractions, backache, shortness of breath and frequent bathroom trips all join the party.
Medically, weeks 37–42 are considered 'term': 37–38 weeks is early term, 39–40 full term, 41 early post-term and 42 post-term. Babies born between 39 and 40 weeks have the best overall outcomes, which is why inductions for non-medical reasons are discouraged before 39 weeks.
The last weeks are about preparation, not prediction: pack the hospital bag around week 34–36, install the car seat by week 36, and treat the due date as a loose center rather than a deadline. Around 60% of babies are born within a week on either side of their due date, and only 4% arrive on the exact day.
How to use our week tracker
Enter your due date and the tracker answers four questions at a glance: your current week and day, your total progress percentage, your trimester, and a rough countdown. Revisit it weekly — or daily in the last month — and pair it with our Due Date Calculator when you need to reverse-engineer a conception window.
A favorite pattern among our users: use the tracker's progress percentage as a small ritual. At 50% (week 20), plan the anatomy-scan celebration; at 75% (week 30), schedule the nursery finishing touches; at 90% (week 36), pack the hospital bag. Milestones are easier to keep when they hang on concrete numbers.
One more honest note: the tracker is calendar math, not a crystal ball. It cannot predict preterm labor, and it cannot detect complications. It exists to orient and organize you — the medical watch belongs to your care team.One question we hear constantly: is it normal that the tracker says week 38 but I feel closer to the end? Yes. Weeks 38–40 are the most physically demanding stretch for many women — the baby has dropped, sleep is interrupted and every day feels like the finish line. The tracker's job at this stage is reassurance and preparation, not precision. Use its countdown as a gentle reminder to rest, and let the care team decide the actual timing of arrival. And if the numbers ever feel abstract, remember the simplest anchor of all: the date your care team gave you at the scan is the one that matters — everything else is supporting cast.
