Reverse Due Date Calculator

This content is provided for educational purposes only. Method from MedlinePlus Prenatal Care.

The Reverse Due Date Calculator computes reverse due date from the relation days from today to LMP = (due date serial day - 280) - today serial day. It takes 6 inputs (due date year, due date month 1 to 12, due date day 1 to 31, today year, today month 1 to 12, today day 1 to 31) and returns the reverse due date. These calculators are for anyone tracking a cycle, a pregnancy or a new baby, and for the midwives, nurses and doctors supporting them, turning recorded dates and measurements into the timing and growth figures used to anticipate what comes next, follow progress against expected patterns, and decide what to raise at the next appointment. Enter your values below and the result updates instantly, and you can share a permalink that pre-fills the exact calculation. These outputs follow directly from the dates and measurements you enter, so use dates exactly as recorded rather than rounded recollections, keep weight and length figures in one unit system where a tool asks for them, and read any single result as less telling than the pattern that builds up over repeated cycles or measurements. For example, with due date year = 2026, due date month 1 to 12 = 10, due date day 1 to 31 = 8, today year = 2025, today month 1 to 12 = 12, today day 1 to 31 = 1, the reverse due date works out to 31, and the worked example further down the page shows every step so you can follow the arithmetic and reproduce it by hand. The method is the standard form documented by MedlinePlus Prenatal Care, and the figure above each result carries the date it was last verified. Cycles, pregnancies and babies all vary widely, and these figures describe typical ranges rather than an assessment of any individual, so discuss anything that worries you, or any result that falls outside what you expected, with your midwife, doctor or pediatrician.