Calculation Methodology & Scientific Formulas
At Pregnancy Calculator, transparency is our core principle. We publish the exact mathematical algorithms, clinical assumptions, and epidemiological literature underlying all calculations.
1. Last Menstrual Period (LMP) Algorithm
Standard obstetric convention establishes human pregnancy at 280 days (40 weeks) from the onset of the LMP. Our algorithm incorporates menstrual cycle calibration:
Rationale: The luteal phase following ovulation remains relatively constant at ~14 days. Variations in cycle length primarily alter the follicular phase prior to ovulation.
2. Conception Date Algorithm
When the exact date of fertilization or assisted reproductive transfer (such as IVF embryo transfer) is known:
Gestational age is standardized by adding 14 days prior to conception to align with medical conventions.
3. Ultrasound Biometric Dating
When dating from an ultrasound scan at a known gestational age (Weeks + Days):
EDD = UltrasoundDate + Remaining Days
4. Trimester Definitions (ACOG Standards)
- First Trimester: Day 0 to 13 weeks 6 days (Days 0 to 97)
- Second Trimester: 14 weeks 0 days to 27 weeks 6 days (Days 98 to 195)
- Third Trimester: 28 weeks 0 days to delivery (Day 196+)
Related Clinical Documentation
Clinical Accuracy & Medical Disclaimer
This calculator provides educational gestational estimates based on standard obstetric formulas (such as Naegele's Rule and 280-day gestational conventions). Only ~4–5% of babies deliver precisely on their calculated due date; 80% deliver within 37 to 42 completed gestational weeks. This tool does not diagnose medical conditions or substitute for prenatal ultrasounds and professional obstetric guidance.