Pregnancy Due Dates and Gestational Age: How Calendar Estimates Work

Pregnancy dates are commonly estimated from the first day of the last menstrual period (LMP). This creates a consistent timeline when the exact day of ovulation or fertilisation is unknown. The estimated due date (EDD) is useful for tracking gestational age and prenatal care, but it does not predict the exact day birth will happen.

The standard LMP estimate

For a usual 28-day cycle, the standard calendar estimate is 280 days, or 40 weeks, from the first day of the LMP. Gestational age also starts on that first day, so it is about two weeks ahead of the estimated conception date.

Why usual cycle length can matter

The 280-day estimate assumes a 28-day cycle. If cycles are consistently shorter or longer, an initial calendar estimate can be adjusted by the difference from 28 days. Irregular cycles, uncertainty about the LMP, assisted reproduction, and clinical findings can all change the best estimate.

What gestational age means

Gestational age is usually written in weeks and days. The first trimester runs through 13 weeks and 6 days, the second from 14 weeks through 27 weeks and 6 days, and the third begins at 28 weeks. These labels describe a timeline; a calculator cannot assess fetal growth, symptoms, or health.

When an ultrasound may be more useful

When the LMP is uncertain or cycles are irregular, an early ultrasound can help establish or confirm gestational age. Professional guidance is important for medical decisions, assisted reproductive technology, symptoms, or any concern about pregnancy health.

Use estimates as a planning tool

• Record the LMP and usual cycle length used.
• Treat the displayed date as an estimate rather than a deadline.
• Share the information with your prenatal-care clinician.
• Seek care for urgent or concerning symptoms rather than relying on a calendar calculator.