Pregnancy Due Date: How It Is Calculated and What to Expect Week by Week

How Your Due Date Is Calculated

Your estimated due date (EDD) is calculated using Naegele's Rule: take the first day of your last menstrual period (LMP), add 7 days, then add 9 months (or subtract 3 months and add 1 year). This assumes a 28-day cycle with ovulation on day 14.

For example, if your LMP was January 1, your estimated due date would be October 8. If your cycle is longer or shorter than 28 days, the due date is adjusted accordingly — a 35-day cycle shifts the date forward by about a week.

Only about 5% of babies arrive on their exact due date. Most are born within a two-week window on either side, and a healthy full-term delivery can occur anywhere between 37 and 42 weeks.

First Trimester (Weeks 1–12)

The first trimester is when the most dramatic development occurs. By week 5, the embryo's heart begins to beat. By week 8, all major organs have started forming. The first trimester is often marked by fatigue, nausea (morning sickness), and breast tenderness.

Key milestones:

Week 6: Heartbeat detectable on ultrasound
Week 9: Embryo is now called a fetus; fingerprints begin forming
Week 12: Risk of miscarriage drops significantly; many parents share the news

Second Trimester (Weeks 13–26)

Often called the "golden trimester" because nausea typically subsides and energy returns. The baby grows rapidly — from the size of a peach to nearly 2 pounds.

Week 16: Baby can hear sounds
Week 18–20: Anatomy scan ultrasound; many parents learn the sex
Week 20: Halfway point — most women feel movement (quickening)
Week 24: Viability milestone — survival outside the womb becomes possible with NICU care

Third Trimester (Weeks 27–40)

The home stretch. The baby gains about half a pound per week, building fat and strengthening lungs for life outside the womb.

Week 28: Eyes can open and close; baby responds to light
Week 32: Baby settles into head-down position (usually)
Week 37: Officially "early term" — lungs are typically mature
Week 39–40: Full term — the ideal window for delivery

Track Your Pregnancy

Use our Due Date Calculator to find your estimated delivery date, and check in weekly with the Pregnancy Week Calculator to see exactly where you are. Track healthy weight gain throughout your pregnancy with the Pregnancy Weight Gain Calculator, and estimate your baby's growth with the Fetal Weight Calculator.