Pregnancy Due Date Calculator is a free browser-based tool that estimates your expected delivery date (EDD) using Naegele's rule, adjusted for your personal cycle length. You enter the first day of your last menstrual period (LMP) and your average cycle length, and the calculator adds 280 days (40 weeks) with a correction for cycles that differ from the standard 28-day assumption. This adjustment makes the estimate more accurate for women with shorter or longer cycles. The tool also shows your current pregnancy week and trimester if your LMP date is in the past, the estimated conception date, and a timeline of key milestone dates including the end of each trimester and the viability milestone at 24 weeks. Pregnancy Due Date Calculator runs entirely in your browser — no personal health data is sent to any server. It is used by expectant parents, healthcare students, and anyone curious about pregnancy timelines. Pregnancy Due Date Calculator is commonly used as a due date calculator pregnancy and a pregnancy week calculator free tool, making it practical for everyday tasks without requiring any software installation. For related tools, Ovulation Calculator can predict your fertile window and ovulation days, and Age Calculator can calculate age and time differences.
Pregnancy dating has been formalized in obstetric practice for over 200 years, with Naegele's rule (named after German obstetrician Franz Karl Naegele, who published his formula in 1812) remaining the standard baseline method. The formula adds 40 weeks to the LMP date, which may seem counterintuitive since conception typically occurs around week 2. This convention exists because the LMP is a reliably known date while conception is not directly observable. The 40-week count therefore includes the approximately two weeks between LMP and ovulation. For women with regular cycles that differ from 28 days, the standard adjustment is straightforward: add one day to the due date for each day the cycle exceeds 28 days, or subtract one day for each day it falls below 28. A woman with a 35-day cycle should expect delivery approximately a week later than the standard Naegele calculation without adjustment. Ultrasound dating, introduced in the 1970s and now universally available in prenatal care in high-income countries, provides more precise gestational age estimates particularly in the first trimester when fetal size is most predictable. Crown-rump length measurement between 7 and 13 weeks has an accuracy of ±5–7 days, which is better than LMP-based estimates for women with irregular cycles. When the LMP-based estimate and ultrasound-based estimate differ by more than a week in the first trimester, the ultrasound date typically takes precedence. Understanding the key milestones in pregnancy — the end of the first trimester at week 13, the anatomy scan around week 20, viability at 24 weeks, and full term at week 37 — helps expectant parents and their healthcare providers plan appropriate care and monitoring at each stage.