What this calculator does
This tool estimates the start and end dates of all three pregnancy trimesters along with your estimated due date (EDD). It works from the first day of your last menstrual period (LMP) and your average menstrual cycle length, the same inputs clinicians use for Naegele's rule.
How to use it
Enter the date of the first day of your last period and your typical cycle length in days (the average is 28). The calculator adjusts the dates if your cycle differs from 28 days, then lists each trimester window and your due date.
The formula explained
A standard pregnancy is counted as 280 days (40 weeks) from the LMP. Because ovulation timing depends on cycle length, every date is shifted by \((\text{cycle} - 28)\) days. The first trimester runs from day 0 to day 97 (week 13 + 6 days), the second from day 98 to day 195 (week 27 + 6 days), and the third from day 196 (week 28) to birth.
$$\begin{gathered} \text{Due Date} = \text{LMP} + \big(280 + \text{adj}\big)\ \text{days} \\[1.5em] \text{where}\quad \left\{ \begin{aligned} \text{adj} &= \text{Cycle Length} - 28 \\ \text{T1 End} &= \text{LMP} + (97 + \text{adj})\ \text{days} \\ \text{T2 End} &= \text{LMP} + (195 + \text{adj})\ \text{days} \\ \text{T3 Start} &= \text{LMP} + (196 + \text{adj})\ \text{days} \end{aligned} \right. \end{gathered}$$
Worked example
For an LMP of 2024-01-01 with a 28-day cycle, the offset is 0. First trimester: 2024-01-01 to 2024-04-07. Second trimester: 2024-04-08 to 2024-07-14. Third trimester begins 2024-07-15, and the due date is 2024-10-07 (280 days later).
FAQ
Is this medically exact? No. It is an estimate; only about 4% of babies arrive on the exact due date. Always confirm dating with your healthcare provider via ultrasound.
What if I don't know my cycle length? Use 28 days, the population average, which leaves the dates unadjusted.
Why do longer cycles push dates later? A longer cycle usually means later ovulation, so conception and every milestone shift forward by the extra days.
