How the due date is actually calculated
Pregnancy is measured from the first day of your last menstrual period (LMP), not from the moment of conception. This 2-week head start exists because most women don’t know the exact moment of conception, but they do know when their last period started.
For a standard 28-day cycle, ovulation happens around day 14, so conception happens 14 days after LMP. From conception to delivery is about 266 days. From LMP to delivery is 266 + 14 = 280 days, or 40 weeks.
The calculator above implements Naegele’s rule: EDD = LMP + 280 days, adjusted for cycle length. If your cycle is 30 days instead of 28, add 2 days. If it’s 26 days, subtract 2 days. The calculator handles this automatically.
The three trimesters
First trimester (weeks 1-12): All major organ systems form. This is the most critical period for fetal development and also when miscarriage risk is highest. Most women experience morning sickness, fatigue and breast tenderness. Schedule first prenatal visit by week 8-10. The NT scan at 11-14 weeks is the first major screening for Down syndrome and other chromosomal conditions.
Second trimester (weeks 13-27): Often the most comfortable phase. Energy returns, morning sickness usually subsides, and the baby starts to move (usually felt around 18-22 weeks). The anomaly scan at 18-22 weeks checks fetal anatomy in detail. Most pregnancies are confirmed publicly during this trimester.
Third trimester (weeks 28-40): Final growth and development. Baby gains the majority of birth weight. Increasingly frequent doctor visits, monitoring of blood pressure, glucose tolerance test (24-28 weeks for gestational diabetes screening), and preparation for delivery. Pack the hospital bag by week 36.
What “due date” doesn’t mean
The due date is a single point estimate of a probability distribution. Only about 5% of babies are born on their exact due date. The full-term window is 37-42 weeks. About:
- 10% of babies arrive before week 37 (preterm)
- 70% arrive in weeks 37-40 (early term and full term)
- 20% arrive in weeks 40-42 (late term)
- Less than 2% go past week 42 (post-term, usually induced)
A more useful framing: by week 40, you have about a 95% chance of having delivered. Your due date is not a deadline, it’s a midpoint estimate.
When LMP and ultrasound dating disagree
If you have an irregular cycle or you can’t remember the exact LMP date, ultrasound dating becomes the reference. First-trimester ultrasound (especially 7-13 weeks) is accurate to within 3-5 days because the embryo grows at a predictable rate during this window.
In the second trimester, ultrasound dating is accurate to within 7-10 days. By the third trimester, it’s only accurate to ±2-3 weeks because individual fetal growth variation increases.
If LMP-based EDD and first-trimester ultrasound EDD differ by more than 7 days, doctors usually adjust to the ultrasound date. The calculator above lets you switch between methods to see the difference.
Cultural and social context in India
In India, the first trimester is traditionally not announced publicly. Many couples wait until after the 12-week NT scan to share the news with extended family, partly because miscarriage risk drops sharply after week 12.
Most Indian hospitals follow international medical guidelines for prenatal care. The typical visit schedule:
| Trimester | Visit frequency | Key tests |
|---|---|---|
| 1st (weeks 1-12) | Once at 6-8 weeks, then monthly | Blood tests, thyroid, dating ultrasound, NT scan at 11-14 weeks |
| 2nd (weeks 13-27) | Monthly | Anomaly scan at 18-22 weeks, glucose tolerance test |
| 3rd (weeks 28-36) | Fortnightly | Growth scans, fetal heart monitoring |
| 3rd (weeks 36-40) | Weekly | Position check, cervical assessment |
Most government hospitals provide all routine tests under the Janani Suraksha Yojana. Private hospital pregnancy care typically costs ₹50,000-₹2,00,000 total depending on city and facility.
When to call the doctor immediately
Outside routine visits, call your obstetrician right away if you experience:
- Heavy vaginal bleeding (not spotting)
- Severe abdominal pain not relieved by rest
- Severe headache with vision changes or blurry vision
- Sudden swelling in hands or face (pre-eclampsia warning)
- No fetal movement for 12+ hours after week 28
- Water breaking before week 37
- Fever above 38°C / 100.4°F
These are the signs that warrant a hospital visit regardless of the time of day.
Calculating from conception instead of LMP
If you tracked ovulation or used IVF, you know the conception date. In that case:
- EDD = conception date + 266 days
- Gestational age = days since conception + 14 days
The calculator handles this when you select “Conception date” as the method. This is the most accurate dating method available, used in IVF pregnancies and when ovulation was confirmed via tracking apps or ovulation test strips.
The post-delivery transition
Once the baby arrives, the gestational age clock stops and “corrected age” begins for premature babies. If your baby is born at 36 weeks instead of 40, doctors use “corrected age” (chronological age minus weeks early) until age 2 for developmental milestones.
For full-term babies, no correction is needed. The due date served its purpose and the rest is parenting.
Related calculators
- Ovulation Calculator for fertility window planning
- Period Calculator for cycle tracking
- BMI Calculator for pre-pregnancy weight status
- Ideal Weight Calculator for pregnancy weight gain targets
Sources
- American College of Obstetricians and Gynecologists (ACOG): Methods for estimating the due date
- WHO: Recommendations on antenatal care for a positive pregnancy experience
- Indian Council of Medical Research (ICMR): Antenatal care protocols
- Naegele’s Rule: standard obstetric dating method, used worldwide