What Is a Due Date Calculator?
A pregnancy due date calculator estimates your Estimated Delivery Date (EDD) — the date your baby is expected to be born. While only around 5% of babies arrive exactly on their due date, the EDD remains a critical medical reference point for tracking fetal development, scheduling prenatal screenings, and planning maternity care. This calculator supports three input methods: Last Menstrual Period (LMP), Conception Date, and IVF Transfer Date.
How Is a Pregnancy Due Date Calculated?
Naegele's Rule (LMP Method)
The most widely used method worldwide is Naegele's Rule, established by German obstetrician Franz Karl Naegele in the 19th century and still the global standard for clinical EDD calculation:
EDD = First Day of LMP + 280 days (40 weeks)
This assumes a 28-day menstrual cycle with ovulation occurring on day 14. For cycles longer or shorter than 28 days, the calculator adjusts the effective LMP date accordingly.
Conception Date Method
If you know your conception or ovulation date, add 266 days (38 weeks) to estimate your EDD. This is equivalent to Naegele's Rule since conception typically occurs 14 days after the start of a 28-day period.
IVF Transfer Date Method
For IVF pregnancies, the calculation depends on embryo age at transfer:
- Day 3 embryo: EDD = Transfer date + 263 days
- Day 5 blastocyst: EDD = Transfer date + 261 days
Pregnancy Trimesters Explained
| Trimester | Weeks | Key Developments |
|---|---|---|
| First Trimester | Weeks 1–13 | Organ formation, heartbeat (week 6), nausea common, miscarriage risk highest |
| Second Trimester | Weeks 14–26 | Fetal movement felt, anatomy scan (week 20), energy returns |
| Third Trimester | Weeks 27–40+ | Rapid fetal growth, lung maturation, preparation for birth |
Key Pregnancy Milestones
- Week 6: Heartbeat detectable via transvaginal ultrasound
- Week 12: End of highest miscarriage risk period; first trimester screening blood tests
- Week 20: Anatomy (morphology) ultrasound scan
- Week 24: Viability milestone (WHO threshold for neonatal care support)
- Week 28: Third trimester glucose tolerance test window
- Week 37: Early term (safe for delivery in most cases)
- Week 39: Full term begins
- Week 40: EDD
- Week 42: Post-term; induction typically recommended
How Accurate Is the Due Date Calculator?
LMP-based calculation has significant inherent variation because not all women have identical 28-day cycles, ovulation timing varies, and implantation windows differ. Studies show that early first-trimester ultrasound (crown-rump length at 8–13 weeks) is more accurate than LMP-based dating and is the clinical gold standard when discrepancy exceeds 7–10 days.
That said, the EDD remains the universally used reference point, and this calculator's output closely matches the standard that your obstetrician or midwife will use.
What If My Due Date Changes After Ultrasound?
It is common for the EDD to be revised after the dating ultrasound, particularly if your cycle is irregular or you are unsure of your exact LMP. If the ultrasound-based date differs from LMP-based date by more than 7–10 days, the ultrasound date is typically used clinically.
Premature, Full Term, and Post-Term Definitions
- Extremely preterm: Before 28 weeks
- Very preterm: 28–32 weeks
- Moderate to late preterm: 32–37 weeks
- Early term: 37–39 weeks
- Full term: 39–41 weeks
- Late term: 41–42 weeks
- Post-term: 42+ weeks
Related Calculators
- Ovulation Calculator — Find your fertile window and peak ovulation days
- BMI Calculator — Pre-pregnancy weight assessment
- Water Intake Calculator — Hydration needs during pregnancy
- Macro Calculator — Nutritional targets during pregnancy