Pregnancy Due Date Calculator

Using the due date estimate

A pregnancy due date is an estimated timing reference, not an appointment the body must follow. This tool starts with the first day of the last menstrual period, adds 280 days and adjusts for the entered cycle length relative to 28 days. It also displays a modeled pregnancy stage relative to the current date. Calendar arithmetic cannot confirm pregnancy, determine clinical gestational age or replace prenatal assessment.

Step-by-step inputs

1. First day of last period. Use the first day of the last menstrual period, not its last day or a positive-test date. An uncertain date makes the estimate less dependable. Starting example: 1969-10-23 .
2. Average cycle length (Advanced options). Enter a representative average cycle length in days. The model adjusts timing by the difference from 28 days and uses a supported range of 20–45 days; it does not measure ovulation. Starting example: 28 days.

Worked example with the starting values

Using the starting inputs, the estimate is Estimated due date: Thu, July 30, 1970: 2,976 weeks pregnant.

The due-date label comes from the last-period date plus the standard duration and cycle adjustment. The weeks-along and days-until figures use the current date, so they change as time passes even if the period input stays the same. The reference is an illustrative calendar scenario, not an assessment of an actual pregnancy.

2976 weeks, 5 days along

Days until due date

0

Trimester

3

Compare average cycle length scenarios

All other inputs stay at their starting values. These are modeled comparisons, not quotes, predictions or individual recommendations.

Make a useful comparison

Check the date source before comparing estimates. Dating from the first period day differs from dating from conception, and the two should not be entered interchangeably. If you know only a test date or have uncertain cycles, ask your prenatal clinician how dating should be established.

Treat a cycle adjustment as an assumption. A longer average cycle shifts the modeled date later, but actual ovulation timing can vary. Irregular cycles and assisted reproduction may require a different dating method rather than a larger adjustment in this form.

Use clinician-established dating for care decisions, screening schedules and questions about pregnancy stage. A prenatal assessment may revise a calendar estimate using other information. The calculator's trimester and week labels are simplified timing aids, not instructions for treatment or delivery planning.

Common mistakes to avoid

• A due date is not a prediction of the exact delivery day. Normal individual timing and clinical circumstances can lead to earlier or later delivery.
• Do not use a weeks-along number to confirm pregnancy or diagnose a problem. The tool relies on the entered date, not a test or clinical examination.
• The comparison table holds the last-period date fixed while changing cycle length. It demonstrates the adjustment rule rather than measuring fetal development or the effect of changing a cycle.