Simple “Rule‑of‑Thumb” Method (LMP)
- Identify the LMP (first day of the last period).
- Add 280 days (40 weeks) to get the estimated due date (EDD).
- GA = (today – LMP) ÷ 7 (weeks) + remainder days.
Quick‑Calc Formula (using dates)
GA_weeks = floor((Today - LMP) / 7)
GA_days = (Today - LMP) % 7
Gestational Age Calculation – Simplified
1️⃣ LMP Method
- Formula: GA = (Today – LMP) / 7 weeks + remainder days
- EDD: LMP + 280 days
2️⃣ Ultrasound Methods
- First‑trimester CRL → lookup table (see below)
- Second‑trimester biometry → average of BPD, HC, FL formulas
3️⃣ Conception‑date Method
- GA = (Today – Conception date) + 2 weeks
Quick‑Reference Tables
- CRL → GA (insert table)
- Biometric formulas (optional)
Example Calculations
- Example 1: LMP = 12 Mar 2025 → GA = 24 w + 1 d (28 Aug 2025)
- Example 2: CRL = 45 mm → GA ≈ 15 w + 0 d
References
- ACOG Practice Bulletin – Dating Pregnancy
- Robinson & Fleming, Obstetric Ultrasound (2022)
1. Last Menstrual Period (LMP) Method
- Formula:
EDD (Estimated Due Date) = LMP + 1 year + 7 days - 3 months - Example:
LMP = May 5, 2024
EDD = May 5, 2024 + 1 year = May 5, 2025
-7 days ≈ April 28, 2025
-3 months ≈ February 28, 2025
GA at present (Oct 1, 2024) = ≈21 weeks
2. Ultrasound Measurement (Crown-Rump Length)
- First-trimester accuracy: GA (days) = CRL (mm) + 42
- Example:
CRL = 50 mm → GA = 50 + 42 = 92 days (≈13.1 weeks)
3. IVF/Assisted Reproduction
- Example:
Embryo transfer date = January 10, 2024
GA = Transfer date + 14 days (ovulation/conception date)
Current GA (Oct 1, 2024) = ≈38 weeks
4. Combined LMP + Ultrasound
- If LMP GA and ultrasound GA conflict:
- <9 weeks: Use ultrasound GA
- 9–16 weeks: Average both
-
16 weeks: Use LMP unless discrepancy >2 weeks
Key Milestones Table:
| GA Range | Development Stage | Clinical Relevance |
|---|---|---|
| <14 weeks | Organogenesis | Confirm viability |
| 18–22 weeks | Anomaly scan | Anatomy assessment |
| >24 weeks | Fetal viability | Resuscitation possible |