Health · 5 min read

How Many Weeks Pregnant Am I? Calculate It Correctly

Pregnancy weeks are counted from the first day of your last menstrual period (LMP) — not from conception. That means by the time you conceive, you are already "2 weeks pregnant" by medical counting.

Why doctors count from your last period

Ovulation dates are uncertain, but most women know when their last period started. So medicine standardised on LMP dating: a 40-week pregnancy includes those first two pre-conception weeks. It feels odd, but every scan, test and milestone chart uses this system. Find your current week instantly with our due date calculator.

Weeks, months and trimesters

• First trimester: weeks 1–12
• Second trimester: weeks 13–26
• Third trimester: weeks 27–40

A "9-month" pregnancy is really 40 weeks ≈ 9 months and 1 week. When someone says "12 weeks pregnant," it means 12 completed weeks — written 12w3d if three extra days have passed.

When the dates get corrected

Your 8–14 week dating scan measures the baby directly and is more accurate than LMP counting — especially if your cycles are irregular or longer than 28 days. If the scan differs from your LMP date by more than a few days, clinicians usually adopt the scan date. Irregular cycles, recent contraceptive use and breastfeeding can all make LMP dating unreliable.

What happens in each trimester

• First trimester (weeks 1–12): major organs and body systems begin forming; morning sickness, fatigue and heightened smell sensitivity are common as hormone levels surge.
• Second trimester (weeks 13–26): often the most comfortable stretch — energy typically returns, the bump becomes visible, and many people feel the baby move for the first time around weeks 18–22.
• Third trimester (weeks 27–40): rapid growth continues, appointments become more frequent, and the body starts preparing for labour — Braxton Hicks contractions are common in the final weeks.

Key milestones by week

Around week 8 the embryo is officially called a fetus. Around weeks 18–22 an anomaly scan checks development in detail. Around week 24 many places consider the pregnancy to have reached "viability," a legally and medically significant milestone. Around week 37 a pregnancy is considered "full term," meaning birth from this point is no longer classed as premature. These are population averages — every pregnancy has its own timeline, and your care provider's guidance always takes precedence over general milestones.

Why your dates might shift during pregnancy

It's common and completely normal for your estimated due date to move slightly as pregnancy progresses. Early ultrasounds (8–14 weeks) measure the baby's crown-rump length, which is a very reliable dating method regardless of when you actually ovulated — this is why the dating scan often takes precedence over an LMP-based estimate. Later scans are used to check growth against the established due date rather than to redate the pregnancy, since fetal size becomes less predictable purely from gestational age as pregnancy progresses.

Tracking your pregnancy week by week

Many people find it helpful to mentally check in every week or two rather than obsessing daily — pregnancy apps and calculators exist to make this effortless. Common things people track include weight gain trends (not a single number, but the trajectory), symptoms to discuss at the next appointment, and simply feeling connected to each stage of development. Whatever method you use, remember that every calculator — including this one — gives an estimate. Fewer than 5% of babies are born exactly on their due date; the vast majority arrive within two weeks either side, which is completely normal.

Appointments and checkups by stage

Early pregnancy typically involves a booking appointment around weeks 8–10 to confirm the pregnancy and discuss initial health screening. The dating and anomaly scans follow at roughly 8–14 weeks and 18–22 weeks respectively. Later pregnancy shifts to more frequent monitoring — often every 2–4 weeks from around week 28, and weekly in the final month — to track growth, position and general wellbeing as the due date approaches. Exact schedules vary by healthcare provider and by whether the pregnancy is considered higher risk, so always follow your own care team's specific plan.

Recap

Pregnancy weeks count from your last period, not conception — check your exact week with the due date calculator, treat the estimated due date as a target window rather than a fixed appointment, and let an early ultrasound refine the dating if your cycle is irregular.

This article is general information, not medical advice — always confirm dates and milestones with your midwife or doctor.

Frequently asked questions