How Is a Pregnancy Due Date Calculated?
The most widely used method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a regular 28-day cycle with ovulation occurring on day 14. If your cycles are irregular, an early ultrasound (6–10 weeks) can provide a more accurate estimated due date (EDD) by measuring the embryo's crown-rump length (CRL).
Pregnancy Trimesters
- First Trimester (Weeks 1–13): All major organs begin forming. The embryo becomes a fetus at week 10. Common symptoms: nausea, fatigue, breast tenderness. Miscarriage risk is highest in this period.
- Second Trimester (Weeks 14–27): Most women feel best during this period — nausea eases, energy returns. The fetus moves noticeably (quickening, typically felt at 16–25 weeks). The 20-week anatomy scan checks all organ systems.
- Third Trimester (Weeks 28–40+): Rapid weight gain and growth. Common discomforts: back pain, frequent urination, Braxton Hicks contractions. Baby is fully developed by week 37 (early term).
Important Disclaimer
Medical Disclaimer: This calculator provides an estimate for informational purposes only. Always confirm your due date with your healthcare provider through clinical examination and ultrasound. Your provider may adjust the EDD based on ultrasound measurements, cycle length, or other clinical factors. Do not make medical decisions based solely on this calculator.
Benefits of Using a Pregnancy Calculator
- Calculates due date via Naegele's Rule: Enter your last menstrual period date and get an estimated due date (EDD) based on the standard 280-day obstetric formula used by healthcare providers worldwide.
- Supports both LMP and gestational age entry: If you already know your gestational age in weeks and days — from an ultrasound — the calculator works backward to give you the same complete picture.
- Identifies your trimester and milestones: See exactly which trimester you are in and upcoming milestones like the anatomy scan window (18–22 weeks), third trimester start, and full-term range (39–40 weeks).
- Private and offline: All calculations run in your browser. No data is stored or sent anywhere.
Frequently Asked Questions
How is the due date calculated?
Naegele's Rule: add 280 days to the first day of your last menstrual period. This assumes a 28-day cycle. An early ultrasound measuring crown-rump length gives a more accurate EDD, especially if your cycles are irregular. Your OB may revise the EDD based on ultrasound measurements.
What are the three trimesters?
First: weeks 1–13 (organ formation, highest miscarriage risk). Second: weeks 14–27 (typically most comfortable, movement begins). Third: weeks 28–40+ (rapid growth, preparation for birth). Full-term is 39–40 weeks; early term is 37–38 weeks.
What is gestational age vs fetal age?
Gestational age counts from LMP. Fetal age counts from conception — typically 2 weeks less. Providers always use gestational age. At 6 weeks gestational age, the embryo is developmentally about 4 weeks old.
What is a full-term pregnancy?
Full-term is 39–40 weeks. Early term: 37–38 weeks. Late term: 41 weeks. Post-term: 42+ weeks. The ACOG revised definitions in 2013, recognizing 39–40 weeks has the best outcomes. Viability threshold is approximately 24 weeks, with survival rates improving significantly after 28 weeks.
When should I take a pregnancy test?
Most accurate from the first day of a missed period (~14 days after ovulation). Testing earlier risks false negatives due to insufficient hCG. Early response tests claim detection 5–6 days before a missed period but accuracy improves significantly closer to the expected period.
What is the 20-week anatomy scan?
Done at 18–22 weeks, it examines all major organs (brain, heart, spine, kidneys, stomach, bladder), checks fetal size and growth, screens for structural anomalies, checks placenta location, and can reveal the sex if parents wish. One of the most detailed prenatal ultrasounds.
What prenatal vitamins should I take?
Start before conception if possible. Key nutrients: folic acid (400–800 mcg/day, prevents neural tube defects), iron (27 mg/day), calcium (1,000 mg/day), vitamin D (600 IU/day), DHA (200–300 mg/day for brain development). Always confirm specifics with your healthcare provider.
What is an Estimated Due Date (EDD)?
EDD is when pregnancy is expected to reach 40 weeks — it's an estimate. Only ~5% of babies arrive on the exact EDD. About 80% of births happen within 2 weeks before or after the EDD. Think of a "due month" rather than a specific date for a more realistic expectation.