Pregnancy Due Date Calculator

How Due Dates Are Calculated

The standard calculation uses Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, conception likely occurred at a different point — an early ultrasound at 8–10 weeks provides a more accurate estimate. Roughly 4% of babies are born on their exact due date. The due date is better understood as the midpoint of a two-week window (39–41 weeks) rather than a precise deadline, and full term is considered 37–42 weeks.

First Trimester: Weeks 1–12

The first trimester is when the most critical development occurs. The neural tube closes by week 6. Heart activity is detectable by ultrasound around week 6–7. By week 12, all major organ systems are formed. Common symptoms include nausea (peaking weeks 6–9), fatigue, breast tenderness, and frequent urination. The first prenatal appointment typically happens at 8–10 weeks and includes a blood panel, Rh factor check, and dating ultrasound. First trimester screening for chromosomal conditions (nuchal translucency ultrasound + bloodwork) occurs at 11–14 weeks.

Second Trimester: Weeks 13–27

The second trimester is generally the most comfortable — nausea subsides for most people and energy returns. The anatomy scan at 18–20 weeks checks fetal development in detail and can reveal sex if desired. Fetal movement (quickening) is typically felt for the first time between weeks 16–22. Glucose screening for gestational diabetes occurs at 24–28 weeks. The fetus reaches viability — survival outside the womb with intensive care — at around week 22–24, though outcomes improve significantly with each additional week.

Third Trimester: Weeks 28–40

In the third trimester, the fetus gains most of its birth weight — about half a pound per week in the final months. Group B Strep testing occurs at 35–37 weeks. The baby typically moves into a head-down position by 36 weeks. Prenatal visits increase to every two weeks at 28 weeks, then weekly after 36 weeks. Signs of labor include regular contractions, water breaking, and loss of the mucus plug. Babies born at 37 weeks are considered full term; 39–40 weeks is optimal for lung maturity and feeding readiness.