Due Date Calculator

An estimated due date provides a standard reference for pregnancy timing, prenatal tests, growth assessment, and delivery planning. Pregnancy is conventionally dated as forty weeks, or 280 days, from the first day of the last menstrual period. This convention begins about two weeks before conception in a typical twenty-eight-day cycle, so gestational age is not the same as fetal age. The last menstrual period method applies Naegele's rule: add 280 days to the first day of the period. The calculator adjusts that estimate when the average cycle differs from twenty-eight days by adding or subtracting the cycle-length difference. This adjustment assumes ovulation occurs about fourteen days before the next period. It may be less reliable when cycles are irregular, the date is uncertain, hormonal contraception was recently stopped, or bleeding was mistaken for a period. When conception is known, the estimate adds 266 days, equivalent to thirty-eight weeks. Even then, fertilization may not occur on the date of intercourse because sperm can survive for several days. Assisted reproduction uses procedure-specific dating rules, and an IVF due date calculator is more appropriate for embryo transfer or egg retrieval. The ultrasound method takes the gestational age measured on the scan date and adds the number of days remaining to forty weeks. First-trimester ultrasound is generally the most accurate imaging method for establishing or confirming gestational age. Later measurements have wider natural variation because fetuses grow at different rates. Clinicians follow guidelines when deciding whether an ultrasound discrepancy is large enough to change an established due date. Once a final estimated due date is documented, repeatedly changing it from later growth scans can create confusion. A due date is an estimate rather than an appointment. Full-term pregnancy spans a range, and only a minority of babies arrive on the exact calculated date. Always rely on the dating recorded by the prenatal care team for medical decisions. Contact a qualified maternity service for bleeding, severe pain, fluid leakage, reduced fetal movement, signs of labor, or any urgent concern. This calculator cannot assess pregnancy viability, fetal growth, or when labor will begin.