Due Date

Due date with a short (24-day) cycle

The adjustment runs both ways. A shorter cycle means earlier ovulation and an earlier estimate.

Worked from fixed dates so the numbers stay put: 2025-01-01 as the last period, read as of 2025-03-15. Use the form on the home page for your own dates.

Estimated due date

Last menstrual period (Naegele’s rule). A reference point, not a prediction — most births are not on this day.

Gestational age on 2025-03-15

11w0d

Counted from the first day of the last period, not from conception.

Days to go

203

Trimester 1

Estimated conception

2025-01-11

An estimate, and not evidence of anything.

Milestones

  • End of first trimester13w6d

    Also the last point at which ultrasound dating is at its most accurate.

  • Anatomy scan window opens18w0d

    The detailed anatomy scan is usually done between 18 and 22 weeks.

  • Anatomy scan window closes22w0d

    After this the standard views become harder to obtain.

  • End of second trimester27w6d

    The third trimester begins the following day.

  • Early term begins37w0d

    ACOG classifies 37w0d to 38w6d as early term, not full term.

  • Full term begins39w0d

    Full term runs 39w0d to 40w6d.

  • Estimated due date40w0d

    A reference point, not a prediction. Most births are not on this day.

  • Post-term begins42w0d

    From 42w0d a pregnancy is classified post-term.

Read before relaying this

  • Read this

    Gestational age counts from the last period, not from conception

    This is the most misunderstood fact in the subject. The 40-week clock starts at the first day of the last menstrual period, roughly two weeks BEFORE conception. So at 11w0d the pregnancy is about 9w0d since conception. Weeks 1 and 2 of a pregnancy are, by this convention, before there is a pregnancy.

    What to do: When comparing against a known conception or transfer date, use that method instead — it does not go through the two-week offset.

  • Read this

    The due date is a reference point, not a prediction

    Only a small minority of births happen on the estimated date. The useful output is the term windows — early term from 37w0d, full term from 39w0d — not the single day.

  • Note

    Adjusted by -4 days for a 24-day cycle

    Naegele’s rule assumes a 28-day cycle with ovulation on day 14. A longer cycle means later ovulation and a later due date; a shorter one, earlier. Most calculators skip this adjustment entirely and are a week out for a 35-day cycle.

  • Note

    Not medical advice

    This is arithmetic on the dates you supplied, not medical advice. It cannot see an ultrasound, a chart, or a person. Clinical dating is done by a qualified clinician, and an estimated due date is an estimate — most births do not happen on it. Nothing here should change what you do without talking to your own clinician.

Other ways to read this page

Available as markdown, or through the API and MCP server.

Related examples

Not medical advice and not a substitute for clinical dating by a qualified clinician. This is arithmetic on the dates you supply; it cannot see an ultrasound, a chart, or a person, and it does not know about multiples, irregular cycles beyond the length you enter, or any complication. Estimated dates are estimates — a small minority of births happen on them.