Pregnancy week calculator

Frequently asked questions

What is a pregnancy due date calculator?

A pregnancy due date calculator is an informational tool that estimates your baby’s arrival date from dates you already know—most often the first day of your last menstrual period (LMP), an estimated conception date, or an ultrasound-based gestational age. It helps you prepare questions for your clinician; it does not replace medical care.

What is Naegele’s rule?

Naegele’s rule is a common dating convention: start from the first day of the LMP and add 280 days (40 weeks). It assumes a 28-day cycle with ovulation around day 14, which is an average—your clinician may adjust dating if your cycles are irregular or if ultrasound dating differs.

How is gestational age calculated?

Gestational age is usually counted from the first day of your LMP unless your care team uses ultrasound-based dating or IVF-specific rules. The calculator converts your inputs into weeks and days from that baseline so you can compare results with clinic paperwork.

How accurate is a due date calculator?

Only a small fraction of babies are born exactly on the estimated due date; many births occur across a broader term window. Ultrasound dating in early pregnancy can reduce error compared with LMP alone, especially when cycles are irregular. Always confirm timing with your obstetric provider.

Can ovulation affect due date estimation?

Yes. LMP-based estimates assume ovulation around the middle of a 28-day cycle. If you ovulate earlier or later, conception-based dating can differ from LMP-based dating. That is one reason clinicians may rely on a first-trimester ultrasound when precise dating matters.