Pregnancy Week Calculator

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How is pregnancy measured in weeks?

Pregnancy is measured in gestational weeks, counted from the first day of the last menstrual period (LMP). This means that at the moment of conception (typically around week 2), you are technically already considered "2 weeks pregnant." A full-term pregnancy lasts 40 gestational weeks, and this is the system used by midwives, obstetricians, and hospitals worldwide. Our calculator tells you how many weeks and days pregnant you are right now, which trimester you are in, and how many weeks remain until your estimated due date.

Why do weeks matter so much in pregnancy?

Each week of pregnancy brings specific developmental milestones, and medical care is calibrated accordingly. The timing of screening tests, scans, and interventions is measured in weeks: for example, the nuchal translucency (NT) scan for Down syndrome screening is performed between 11+0 and 13+6 weeks; the anatomy scan at 18–20 weeks; Group B Streptococcus (GBS) testing around 35–37 weeks. Knowing your exact gestational age ensures you receive the right care at the right time.

Week-by-week highlights

In the first trimester (weeks 1–13), all major organ systems are formed. The heart begins beating around week 6; fingers and toes are distinct by week 10; by week 12, the baby is fully formed and growing. In the second trimester (weeks 14–26), the baby grows rapidly and movements begin to be felt, usually between weeks 18 and 22 for first pregnancies. In the third trimester (weeks 27–40+), the baby gains fat, the lungs mature, and the baby descends into the pelvis in preparation for birth.

What does "weeks + days" notation mean?

Healthcare providers often write gestational age as "X+Y" — for example, "28+3" means 28 weeks and 3 days pregnant. This precision matters because some decisions (like the timing of steroid injections for lung maturity in preterm labour) depend on the exact day. Our calculator shows your pregnancy in this standard format so you can communicate clearly with your medical team.

What if my dates do not match the scan?

If an ultrasound scan gives a gestational age that differs from your LMP-based calculation, the scan date is usually considered more accurate, particularly in the first trimester. Your healthcare provider may adjust your due date based on the scan, which will change your week count accordingly. This is normal and common, particularly in people with irregular cycles or uncertain LMP dates.