Gestational Age Calculator 2025: LMP, Conception, Ultrasound, and Due Date
Gestational age (GA) measures pregnancy from the first day of the last menstrual period (LMP). Our calculator uses LMP, conception, or ultrasound to determine exact fetal age in weeks and days, due date, and trimester.
Why Gestational Age Matters
GA guides prenatal testing, growth monitoring, and delivery planning. Accurate dating reduces preterm/post-term risks.
Calculation Methods
1. LMP Method (Naegele’s Rule)
EDD = LMP + 280 days (40 weeks). Assumes 28-day cycle, ovulation on day 14.
Formula: GA = Today − LMP
2. Conception Date
GA = Today − Conception + 14 days (adds 2 weeks from LMP to ovulation).
3. Ultrasound
- CRL (6–13 weeks): Most accurate (±3–5 days)
- BPD (14+ weeks): ±7–10 days
Trimester Breakdown
| Trimester | Weeks | Milestones |
|---|---|---|
| First | 0–12 | Organ formation, NT scan |
| Second | 13–27 | Anatomy scan, quickening |
| Third | 28–40 | Growth, kick counts |
Step-by-Step Example
LMP: March 1, 2025. Today: June 15, 2025.
- Days since LMP = 106
- GA = 15 weeks 1 day
- EDD = March 1 + 280 = December 6, 2025
Accuracy by Method
- Early US (≤13w): ±5 days
- LMP (regular cycles): ±7 days
- Late US: ±14–21 days
Clinical Use
- Schedule anatomy scan at 18–20 weeks
- Start kick counts at 28 weeks
- Plan induction if >41 weeks
Limitations
- Irregular cycles skew LMP
- Late ultrasound less accurate
- IVF: use embryo transfer date + 14 days
Integration with Tools
FAQs
Conception vs gestational age? Conception age = GA − 2 weeks.
Can I use after 20 weeks? Yes, but early US is gold standard.
What if cycles are irregular? Rely on ultrasound.
Conclusion
Accurate gestational age is the foundation of prenatal care. Use our 2025 calculator with early ultrasound for best outcomes. Always confirm with healthcare provider.
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