How are pregnancy due date and gestational age calculated?
The most common starting point is the first day of the last menstrual period (LMP). The traditional estimated due date is 280 days, or 40 weeks, after that date. Hesapica also supports a known conception date and IVF embryo transfer as separate dating methods.
Which dating method should I use?
If the LMP is known reliably, the LMP mode is a practical starting point. If conception date is known, the calculator uses 266 days. For IVF pregnancies, ACOG recommends ART-derived dating based on embryo age and transfer date: a day-5 embryo corresponds to 261 days after transfer and a day-3 embryo to 263 days.
Is gestational age the same as fetal age?
No. Clinical gestational age traditionally begins on the first day of the LMP, roughly two weeks before conception in a typical 28-day cycle. The pregnancy week shown here can therefore be about two weeks greater than time since fertilization.
How exact is the estimated due date?
An EDD is a reference date, not a guaranteed delivery day. ACOG notes that only about 1 in 20 births occur on the exact estimated date. First-trimester ultrasound is the most accurate method to establish or confirm gestational age, and clinical dating should take priority over a web calculator.
What does the cycle-length adjustment do?
The classic 280-day rule assumes a regular 28-day cycle with ovulation around day 14. For consistently longer or shorter cycles, this tool applies a simple calendar adjustment. It cannot correct for irregular cycles, uncertain LMP recall or variable ovulation.
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Frequently asked questions
Is the calculated due date exact?
No. It is an estimate used for pregnancy dating. ACOG notes that only about 1 in 20 women give birth on the exact estimated due date.
Why does the LMP method use 280 days?
By convention, pregnancy is dated as 40 weeks or 280 days from the first day of the last menstrual period.
Why can cycle length change the LMP estimate?
The conventional 280-day rule assumes a 28-day cycle and ovulation around day 14. A simple cycle-length adjustment can make a calendar estimate more representative for consistently longer or shorter cycles, but it does not replace ultrasound dating.
How is IVF transfer handled?
The calculator estimates the fertilization date by subtracting the embryo age at transfer, then adds 266 days from fertilization.
Can this calculator replace an ultrasound or clinician?
No. First-trimester ultrasound is generally the most accurate way to establish or confirm gestational age when dating is uncertain.