Pregnancy Calculator Guide: Due Dates, Trimesters, and Milestones

Calculating Your Due Date

Naegele's rule: Add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Most pregnancies deliver within 2 weeks of this date in either direction. Only about 5% of babies are born on their exact due date.

Alternative calculation: LMP + 9 months + 7 days = estimated due date.

The Three Trimesters

First trimester (Weeks 1–12): Most critical for organ development. Morning sickness, fatigue, and emotional changes are common. Miscarriage risk highest in this period (10–20% of known pregnancies).
Second trimester (Weeks 13–26): Most women feel best during this trimester. Nausea typically subsides, energy returns. Fetal movements ("quickening") felt around weeks 18–22. Anatomy ultrasound typically at weeks 18–20.
Third trimester (Weeks 27–40): Rapid fetal weight gain. Braxton Hicks contractions, back pain, difficulty sleeping common. Baby considered full-term at 39 weeks.

Key Prenatal Screening Timeline

First prenatal visit (8–10 weeks): Blood work, ultrasound, medical history
First trimester screen (11–13 weeks): Nuchal translucency ultrasound + blood markers
Cell-free DNA (NIPT): 10+ weeks, screens for chromosomal abnormalities
Anatomy scan (18–20 weeks): Detailed structural ultrasound
Glucose tolerance test (24–28 weeks): Screens for gestational diabetes
Group B Strep test (35–37 weeks)

Frequently Asked Questions

How accurate is a due date calculator?

Due date calculators using LMP provide an estimate — only about 5% of babies arrive on their exact due date. Early ultrasound (8–12 weeks) is the most accurate dating method, adjusting the due date based on fetal measurements. Most providers use ultrasound measurements if more than 5–7 days off from LMP-based calculation.

What is gestational age vs. fetal age?

Gestational age counts from the first day of your last period (2 weeks before ovulation). Fetal age counts from conception (roughly 2 weeks later). Doctors use gestational age as the standard. A 10-week gestational age fetus is actually approximately 8 weeks old from conception.