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Pregnancy age, due date and cycle — what do the dates mean?

Pregnancy and cycle dates are estimates based on conventions and assumptions. They may support a clinician discussion but do not confirm ovulation, conception or healthy pregnancy development.

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Simple Tools editorial teamReviewed: 22 September 2026

Gestational age begins before conception

Gestational age is normally counted from the first day of the last menstrual period. It is therefore commonly about two weeks greater than time since conception.

With irregular cycles or an uncertain date, the difference may be larger and early ultrasound can change clinical dating.

A due date is a statistical estimate

The due-date tool distinguishes last-period dating with a regular-cycle adjustment, known fertilisation and day-3 or day-5 IVF transfers. Gestational age can be advanced from a date or assessment already established in prenatal care, without interpreting ultrasound measurements.

A clinician considers cycle history, examinations and pregnancy development and may set a different working date.

A fertile window does not confirm ovulation

The scenario assumes ovulation on cycle day equal to cycle length minus 14, with the first bleeding day counted as day 1. Each six-day window ends on the modelled ovulation day; cycle variability prevents these dates from being certain.

Do not use the calculator as contraception or confirmation of fertility. Those require methods and advice appropriate to the situation.

Frequently asked questions

What does this guide explain about “Gestational age begins before conception”?

Gestational age is normally counted from the first day of the last menstrual period. It is therefore commonly about two weeks greater than time since conception. With irregular cycles or an uncertain date, the difference may be larger and early ultrasound can change clinical dating.

What does this guide explain about “A due date is a statistical estimate”?

The due-date tool distinguishes last-period dating with a regular-cycle adjustment, known fertilisation and day-3 or day-5 IVF transfers. Gestational age can be advanced from a date or assessment already established in prenatal care, without interpreting ultrasound measurements. A clinician considers cycle history, examinations and pregnancy development and may set a different working date.

What does this guide explain about “A fertile window does not confirm ovulation”?

The scenario assumes ovulation on cycle day equal to cycle length minus 14, with the first bleeding day counted as day 1. Each six-day window ends on the modelled ovulation day; cycle variability prevents these dates from being certain. Do not use the calculator as contraception or confirmation of fertility. Those require methods and advice appropriate to the situation.

Calculators covered by this guide

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