Pregnancy Calculator
Enter your last menstrual period (LMP) or conception date to calculate your due date, current gestational age, trimester, and key milestones.
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Frequently Asked Questions
What happens each trimester?
First trimester (weeks 1–12): organ formation, highest miscarriage risk. Second trimester (weeks 13–26): fetus grows rapidly, movement felt. Third trimester (weeks 27–40): lungs mature, baby gains weight and turns head-down.
When do pregnancy symptoms typically start?
Most common first symptoms appear at 4–6 weeks: nausea, fatigue, breast tenderness, and frequent urination. Some women notice implantation spotting at 6–12 days after conception.
What prenatal tests are done each trimester?
First trimester: blood type, CBC, genetic screening (NIPT), nuchal translucency ultrasound. Second trimester: anatomy ultrasound (18–20 weeks), glucose challenge test. Third trimester: Group B strep test, fetal position check, NST if overdue.
Pregnancy Calculator: How Gestational Age Is Measured and What It Means
Pregnancy is measured in weeks, and understanding how those weeks are counted is one of the first things expectant parents encounter at their initial prenatal appointment. The gestational age system used by obstetricians worldwide starts counting from the first day of the last menstrual period — not from the date of conception — which means pregnancy is technically dated from a point before fertilisation actually occurred. This counterintuitive convention has a practical basis and shapes everything from due date calculations to prenatal screening schedules.
The LMP Method: Why Pregnancy Weeks Start Before Conception
Before the advent of ultrasound, the first day of the last menstrual period was the only reliably known date in early pregnancy. Conception typically occurs around two weeks after the LMP during ovulation, but the exact date of ovulation — let alone fertilisation — is rarely known with precision. By anchoring gestational age to the LMP, clinicians have a consistent reference point that every patient can report. Full-term pregnancy is therefore counted as 40 weeks from the LMP, even though actual foetal development begins at around week 2 or 3.
This convention means that when your doctor says you are eight weeks pregnant, approximately six weeks have passed since conception. The terminology can feel confusing at first, but it aligns with how all obstetric reference charts and fetal development milestones are reported. When comparing your pregnancy calculator result to any medical resource, the gestational age will match the LMP-based calculation.
The Three Trimesters
Pregnancy is conventionally divided into three trimesters, each covering roughly 13 weeks. The first trimester spans weeks 1 through 13 and encompasses the period of most rapid embryonic development — all major organ systems form during this window. The risk of miscarriage is highest in the first trimester, which is why many people choose to wait until after week 12 to announce their pregnancy. The first trimester is also when morning sickness and fatigue are typically most pronounced.
The second trimester, weeks 14 through 27, is often described as the most comfortable phase of pregnancy. The nausea of early pregnancy typically subsides, energy returns, and the pregnancy becomes visible. Fetal movements — quickening — are first felt around weeks 18 to 22. Anatomy scans performed around week 20 provide a detailed look at fetal development and are often when the sex of the baby can be determined. The third trimester, weeks 28 through 40 (and beyond), is characterised by rapid fetal weight gain and preparation for birth.
How Accurate Is the Calculated Due Date?
The due date calculated from the LMP is an estimate, not a prediction. Only about 4–5% of babies are born on their exact due date. The vast majority — roughly 80% — are born within two weeks on either side of the calculated date. A normal full-term pregnancy can end anywhere between 37 and 42 weeks of gestation. Babies born before 37 weeks are considered premature; those born after 42 weeks are considered post-term and are typically managed medically to avoid complications from placental ageing.
Ultrasound measurements taken in the first trimester, particularly between 8 and 13 weeks, can refine the due date estimate significantly. If the LMP-based calculation and the first-trimester ultrasound differ by more than five days, most obstetricians will revise the due date to match the ultrasound measurement, which is generally more accurate at that stage. Cycle irregularity, recent hormonal contraceptive use, and conditions like polycystic ovary syndrome (PCOS) can all cause the LMP-based calculation to be less accurate, making early ultrasound dating especially important.
Using Your Pregnancy Calculator Result
Your pregnancy calculator gives you a starting point for understanding where you are in your pregnancy and when key milestones and appointments typically occur. Use it as a guide to prepare questions for your healthcare provider, to understand the context of prenatal screening results, and to track your progress week by week. Every pregnancy is unique, and the most important source of personalised guidance is your midwife, obstetrician, or family doctor — the calculator provides context and education, not medical advice.