Pregnancy Calculator

About This Tool

The Pregnancy Calculator uses Naegele's rule, the standard method used by healthcare providers worldwide to estimate due dates. Named after German obstetrician Franz Naegele, who published the formula in 1812, this method adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). While seemingly simple, this approach has been validated over two centuries of clinical use and remains the starting point for all pregnancy dating.

How Due Date Calculation Works

Gestational age is counted from the first day of your last menstrual period, even though conception typically occurs about 14 days later (around ovulation). This means that during “weeks 1 and 2” of pregnancy, you are not actually pregnant yet — the convention exists because LMP is a reliably known date while conception timing is often uncertain. A full-term pregnancy is 37-42 weeks from LMP, with 40 weeks being the “due date.” Most babies are born between 38 and 42 weeks, and only about 5% arrive on their exact due date.

Understanding Trimesters

The first trimester (weeks 1-12) is the period of most rapid development: all major organs form, the heart begins beating, and the embryo transitions to a fetus by week 10. This is also when morning sickness peaks. The second trimester (weeks 13-26) is often called the “golden period” as nausea typically subsides, energy returns, and you can feel the baby move (quickening) around weeks 16-20. The third trimester (weeks 27-40) focuses on brain development, lung maturation, and rapid weight gain as the baby prepares for birth.

Key Prenatal Screenings

Several important screenings are scheduled throughout pregnancy. The first trimester screen (weeks 11-14) combines a nuchal translucency ultrasound with blood work to assess risk of chromosomal conditions like Down syndrome. The anatomy scan (weeks 18-22) is the detailed ultrasound that checks all major organs and structures. The glucose tolerance test (weeks 24-28) screens for gestational diabetes, which affects 6-9% of pregnancies. The Group B Strep test (week 36) determines whether antibiotics are needed during labor to protect the baby.

Factors That Affect Due Dates

Your provider may adjust your due date based on first-trimester ultrasound measurements, which are accurate to within 5-7 days. Later ultrasounds are less accurate for dating. Due dates may also be adjusted for IVF pregnancies (where conception date is precisely known), irregular menstrual cycles (where the LMP assumption of ovulation at day 14 may not hold), and first vs. subsequent pregnancies (first babies tend to arrive a few days later on average).