Pregnancy Calculator — Due Date, Trimesters, and What Each Week Actually Means

You found out you are pregnant. The first question everyone asks: "When are you due?" You do some quick math — it is June, nine months from now is March, so… March? Then your doctor tells you the due date is actually calculated from your last menstrual period, not from conception, and suddenly your mental math is off by two weeks.

Our free pregnancy calculator gives you the due date, current week, trimester, and key milestones. Here is why the math works the way it does and what each stage actually means.

Why 40 weeks, not 9 months

Pregnancy is 40 weeks from the first day of your last menstrual period (LMP). Conception typically happens around week 2 of this count — during ovulation, roughly 14 days after the period started. So the first two weeks of "pregnancy" you are not actually pregnant yet. This sounds absurd, but it is the standard because the LMP date is usually known, while the exact conception date is usually not.

This means when a pregnancy test shows positive at "4 weeks pregnant," the embryo has only existed for about 2 weeks. The math is confusing by design — it is a dating convention, not a biological measurement.

The three trimesters: what actually happens

First trimester (weeks 1-13): all major organs and body systems form. The heart starts beating around week 6. By week 12, the fetus has fingers, toes, and a profile that looks recognizably human. This is also when miscarriage risk is highest (about 10-20% of known pregnancies), which is why many people wait until week 12 to announce.

Second trimester (weeks 14-27): the "golden period" for most people. Nausea typically subsides, energy returns, and the baby's movements become noticeable around weeks 18-22. The anatomy scan at week 20 checks all major structures and can usually determine sex if desired.

Third trimester (weeks 28-40): rapid weight gain, lung maturation, and preparation for birth. Babies born after 37 weeks are considered "full term." Babies born before 37 weeks are preterm and may need NICU support depending on how early they arrive.

Due date accuracy: it is an estimate

Only about 4% of babies arrive on their exact due date. The due date is the midpoint of a 5-week window: anytime between 37 and 42 weeks is considered "term." First-time mothers average 41 weeks + 1 day. The due date is useful for planning — when to stop working, when family should visit, when to schedule the last prenatal appointments — but it is not a deadline. The baby did not read the calendar.

Factors that shift the due date: irregular cycles make LMP-based dating less accurate (early ultrasound dating is more reliable), IVF pregnancies use the transfer date for precise dating, and twins typically arrive earlier (around 36-37 weeks on average).

Our pregnancy calculator gives you the standard 40-week estimate based on LMP. For more precise dating, your healthcare provider will use first-trimester ultrasound measurements, which are accurate to within 5-7 days.

For calculating age after birth, our age calculator gives exact years, months, and days. And for health metrics during pregnancy, our BMI calculator provides pre-pregnancy BMI context.