How does a due date calculator work?
A due date is calculated as 280 days (40 weeks) from the first day of your last menstrual period, often called LMP. This is the standard formula used by most doctors and midwives worldwide. It assumes ovulation happens around day 14 of your cycle, which is true for most women with regular 28-day cycles.
When does my due date fall?
Only about 5% of babies arrive on the exact estimated due date. About 80% arrive within two weeks before or after. So your due date is a strong target, not a deadline. First-time pregnancies tend to go slightly past, while subsequent pregnancies tend to arrive a bit sooner.
What if my cycles are irregular?
If your cycle varies a lot or you cannot remember your last period date, an early ultrasound (before week 13) is the most accurate way to estimate gestational age. Your doctor will measure the size of the embryo and calculate the due date from there.
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