Practical guide
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.
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 280-day last-period or 266-day conception rule gives an indicative date, not a guaranteed delivery day.
A clinician considers cycle history, examinations and pregnancy development and may set a different working date.
A fertile window does not confirm ovulation
A calendar model subtracts an assumed luteal phase from cycle length. Stress, illness and natural variation can shift ovulation.
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 280-day last-period or 266-day conception rule gives an indicative date, not a guaranteed delivery day. 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”?
A calendar model subtracts an assumed luteal phase from cycle length. Stress, illness and natural variation can shift ovulation. 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
Pregnancy week calculator
Estimate gestational age on a selected date from the last menstrual period.
Estimated due date calculator
Estimate a due date from the last menstrual period or conception date.
Fertile window calculator
Produce only an indicative fertile window from cycle length assumptions.