How Many Weeks Pregnant Am I?

This is the same calculation as our Gestational Age Calculator, framed conversationally. Enter your last menstrual period to find out how many weeks and days pregnant you are.

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Why You Are "4 Weeks Pregnant" at a Missed Period

This is the most confusing part of pregnancy dating, and it surprises almost everyone.

Pregnancy is counted from the first day of your last menstrual period (LMP), not from the date you conceived. This means:

  • At the moment of conception, you are already considered 2 weeks pregnant.
  • At the time of a missed period, you are considered 4 weeks pregnant.
  • Your due date is 40 weeks from your LMP, which is about 38 weeks from conception.

This is not arbitrary. It is the standard recommended by ACOG and used by healthcare providers worldwide. The reason is practical: most people know the date of their last period, but very few know the exact date of conception.

The Calculation

Gestational age = Today's date − (LMP + cycle adjustment)

The cycle adjustment accounts for cycles that are not exactly 28 days. If your cycle is 32 days, for example, ovulation likely occurred about 4 days later than the standard assumption, so your due date is adjusted forward by 4 days.

What the Number Means

Gestational age is expressed in weeks and days. For example:

  • 4+0 means 4 weeks and 0 days pregnant
  • 22+3 means 22 weeks and 3 days pregnant
  • 39+6 means 39 weeks and 6 days pregnant — the last day of the 39th week

How Accurate Is This?

LMP-based dating is reasonably accurate if:

  • Your cycles are regular
  • You know the date of your last period
  • You are not currently breastfeeding or have recently stopped hormonal contraception (both can affect cycle regularity)

A first-trimester ultrasound is more accurate than LMP-based dating. If the two differ by more than 7 days, ACOG recommends using the ultrasound date.

When to Talk to Your Clinician

Contact your healthcare provider if:

  • You do not know the date of your last period
  • Your cycles are irregular
  • You have bleeding, pain, or other concerns
  • You want to confirm your due date with an ultrasound
Not medical advice: This tool provides an estimate based on standard formulas. It does not replace professional medical advice, diagnosis, or treatment. Always consult your own clinician.
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