A pregnancy due date calculator estimates when your baby is expected to be born based on the first day of your last menstrual period (LMP) and your cycle length. The standard calculation adds 280 days (40 weeks) to your LMP — a method called Naegele's rule — and adjusts for cycles that are longer or shorter than the typical 28 days. Women with a 35-day cycle, for example, ovulate later and their due date shifts approximately one week later compared to the standard estimate. Enter your LMP date and average cycle length to get your estimated due date, along with the current pregnancy week and trimester if you are already pregnant. The tool also shows a timeline of key milestone dates: the end of the first trimester at week 13, the anatomy scan window around week 20, the viability threshold at week 24, and the end of the second trimester at week 27. These milestones are useful for planning prenatal appointments and understanding what stage of fetal development to expect at each point. The calculator runs entirely in your browser — no personal health data is ever sent to a server. Results are estimates for planning purposes; your healthcare provider may establish a more precise due date from a first-trimester ultrasound.
The estimated due date (EDD) is one of the most important numbers in obstetric care, but it is important to understand what it represents and what it does not. The EDD is a statistical midpoint: based on large population studies, it is the date by which 50% of pregnancies will have delivered. Only about 4–5% of babies are actually born on their exact due date. The vast majority of births — approximately 80% — occur within two weeks before or after the EDD. Clinically, a birth is considered "on time" if it occurs between 37 and 42 weeks. The EDD is used to schedule prenatal screening tests at appropriate gestational ages, to track fetal growth against expected norms, to time interventions like elective induction, and to identify post-term pregnancies that may require additional monitoring. Dating accuracy matters because gestational age affects the interpretation of every prenatal test. An alpha-fetoprotein screening test at 17 weeks has a different normal range than the same test at 19 weeks. Nuchal translucency measurements are only valid within a narrow window in the first trimester. If gestational age is off by even 10 days, screening results can be misinterpreted. This is why first-trimester ultrasound confirmation of gestational age is considered best practice. The two-week "buffer" between conception and the start of LMP counting occasionally confuses first-time parents. When a provider says you are "6 weeks pregnant," your embryo has actually been developing for approximately 4 weeks since conception. This convention exists purely because the first day of the last period is a reliably known date while the exact moment of conception typically is not. The 40-week count from LMP has been the standard in clinical obstetrics for over 200 years, predating modern understanding of fertilization, and remains universal because it produces consistent, comparable results across all pregnancies.