Pregnancy Due Date Calculator (Estimated Delivery Date)
Estimate your baby's due date, track current gestational weeks and days, identify your trimester stage, and review key fetal developmental milestones using clinical obstetric standards (Naegele's Rule).
How Obstetric Due Dates Are Calculated: Naegele's Rule
In obstetrics, the standard clinical method for determining the Estimated Delivery Date (EDD) is Naegele's Rule, formalized by German obstetrician Franz Karl Naegele in the early 19th century. The calculation assumes a regular 28-day human menstrual cycle with ovulation and ovum fertilization occurring precisely on cycle day 14.
Or simply: LMP + 280 calendar days (40 completed obstetric weeks / 10 lunar months).
Adjusting for Irregular Cycle Lengths
Women with menstrual cycle intervals differing from the textbook 28 days ovulate earlier or later than day 14. Our algorithm automatically adjusts the calculation window:
- For a 24-day cycle: Ovulation occurs around Day 10 (EDD is adjusted 4 days earlier).
- For a 32-day cycle: Ovulation occurs around Day 18 (EDD is adjusted 4 days later).
The 3 Trimesters of Fetal Development
| Trimester | Gestational Weeks | Major Embryonic & Fetal Milestones |
|---|---|---|
| First Trimester | Weeks 1 – 12 | Organogenesis: Neural tube closure, primitive cardiac contractions (week 5–6), limb buds, facial structure formation. |
| Second Trimester | Weeks 13 – 27 | Rapid growth: Gender anatomical differentiation visible on 20-week anatomy scan, quickening (fetal movement felt), hearing develops. |
| Third Trimester | Weeks 28 – 40+ | Maturation: Pulmonary alveolar surfactant synthesis, substantial subcutaneous fat deposition, head engagement into the maternal pelvis. |
Clinical Definitions of Pregnancy Term Categories (ACOG Guidelines)
The American College of Obstetricians and Gynecologists (ACOG) classifies deliveries between 37 and 42 weeks into four clinical categories:
- Early Term: 37 weeks, 0 days through 38 weeks, 6 days.
- Full Term: 39 weeks, 0 days through 40 weeks, 6 days (lowest statistical rate of neonatal respiratory complications).
- Late Term: 41 weeks, 0 days through 41 weeks, 6 days.
- Postterm: 42 weeks, 0 days and beyond (requires close biophysical profile monitoring).