The last-period estimate

A common method counts 40 weeks, or 280 days, from the first day of the last menstrual period. It is a calendar convention, not a prediction that birth will happen on that day.

  • The estimate assumes a typical cycle; individual cycles vary.
  • If you are unsure of the date or have irregular cycles, ask your care team how to date the pregnancy.

A calendar example

Using the usual last-period method, January 4, 2027 plus 280 days gives October 11, 2027. An online result is an estimate and does not replace the date your clinician documents.

  • Count from the first day of the period, not its last day.
  • Use the due date documented by your care team for medical appointments.

Ultrasound and IVF dates

A care team may use the first accurate ultrasound and other information when establishing gestational age. For IVF, embryo age and transfer date are used for dating; your fertility team can confirm which details apply.

  • Ask your team why its recorded date differs from an online estimate.
  • For IVF, do not substitute the date an embryo was frozen for the transfer date.

What to do with different dates

Write down the date used by your healthcare team and ask before changing a date used for your care. The Niva calculator can help you understand the math but cannot establish your clinical due date.

  • Keep your appointments as advised by your care team.
  • Use the week calculator to explore an estimate, not to decide when to seek care.