How to Calculate Conception Date, Gestational Age, and Pregnancy Due Date
To calculate your estimated conception date, add 14 days to the first day of your last menstrual period (LMP) for a standard 28-day cycle, or calculate backward 266 days (38 weeks) from your clinical due date. Clinical gestational age is measured from the LMP rather than the conception date, meaning you are considered 2 weeks pregnant at the time of fertilization.
Key Takeaway Facts
- Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP).
- Human fertilization occurs within 12 to 24 hours after ovulation during the peak fertility window.
- Sperm can survive in the female reproductive tract for up to 5 days preceding ovulation.
- First-trimester crown-rump length (CRL) ultrasound measurements have an accuracy margin of ±5 to 7 days.
- Fetal age is exactly 14 days less than clinical gestational age in standard cycles.
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1. Clinical Gestational Age vs Embryonic Fetal Age
Frequently Asked Questions
Can you know the exact day you got pregnant?
Unless pregnancy was achieved through assisted reproductive technology (such as IVF embryo transfer) or sexual intercourse occurred only once during the entire cycle, identifying the exact calendar day with 100% certainty is challenging because viable sperm can fertilize an egg up to 5 days after intercourse.
Editorial Review & Fact-Checking Assurance
This guide was researched and drafted by the ToolQix Clinical Health Advisory Panel and technically verified by Board-Certified OB/GYN Physician under ToolQix's strict accuracy protocols. Formulas, calculations, and instructions were independently tested against current industry specifications.
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