Pregnancy Calculator Guide: Due Date, Weeks & Trimesters
Understanding how pregnancy dating works — estimated due dates, gestational weeks, and trimesters — helps expectant parents track their pregnancy accurately.
Naegele's Rule for Due Date
Or: LMP + 9 months + 7 days
Add 9 months = 1 October 2025
Add 7 days = 8 October 2025 (estimated due date)
Pregnancy Timeline by Trimester
Organ formation, heartbeat (week 6), brain development, highest risk period
Rapid growth, movement felt (week 18–20), gender visible on ultrasound, most comfortable trimester
Lung maturation, rapid weight gain, baby moves into head-down position, preparation for birth
Frequently Asked Questions
How is the estimated due date (EDD) calculated?
EDD = LMP date + 280 days (40 weeks). Naegele's Rule: EDD = LMP + 9 months + 7 days. For example, LMP on 1 January: EDD ≈ 8 October. This assumes a regular 28-day cycle. Ultrasound measurements in the first trimester are more accurate.
What is LMP?
LMP stands for Last Menstrual Period — the first day of your last period before becoming pregnant. Gestational age (weeks pregnant) is counted from LMP, not from conception. So at the time of conception (around week 2), you are technically 2 weeks pregnant.
How many weeks are in a full-term pregnancy?
A full-term pregnancy is 39–40 weeks. Early term: 37–38 weeks. Full term: 39–40 weeks. Late term: 41 weeks. Post-term: 42+ weeks. Babies born before 37 weeks are considered premature (preterm).
What are the three trimesters of pregnancy?
First trimester: weeks 1–12 (organ formation, highest miscarriage risk). Second trimester: weeks 13–26 (growth phase, usually most comfortable). Third trimester: weeks 27–40 (rapid weight gain, preparation for birth). Most prenatal tests are trimester-specific.
Can I get an accurate due date from ultrasound?
Yes — first-trimester ultrasound (7–13 weeks) is the most accurate way to determine gestational age, with a margin of ±5–7 days. Second-trimester ultrasound has a margin of ±2 weeks. If ultrasound differs significantly from LMP date, doctors typically use the ultrasound date.
Related Calculators
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