About this tool

Ultrasound vs LMP Due Date Comparator

Compare the due date from your last period with the scan-dated one and see whether the gap crosses the ACOG redating threshold.

This comparator puts two due dates side by side: the one from Naegele's rule (last period + 280 days, shifted for a cycle that is not 28 days) and the one implied by the gestational age your ultrasound reported. It measures the gap in days and checks it against the ACOG redating thresholds — 5 days up to 8w6d, 7 days to 15w6d, 10 days to 21w6d, 14 days to 27w6d and 21 days from 28 weeks — so you can see whether a clinician would normally keep the period date or adopt the scan date.

How to use Ultrasound vs LMP Due Date Comparator

1. 1

   Open Ultrasound vs LMP Due Date Comparator on AltFTool — it loads instantly in your browser.

2. 2

   Enter the values you already know.

3. 3

   Fine-tune the options to match your scenario.

4. 4

   Read the result and use it in your planning or reporting.

Why use Ultrasound vs LMP Due Date Comparator

• Both methods, one screen

  Naegele's rule with a cycle-length correction, and the scan-implied date, computed the same way a clinic would.

• Threshold-aware

  Applies the published redating cut-offs instead of leaving you to judge whether a gap is big.

• Explains the gap

  Shows the gestational age each method gives on the scan day, not just the two end dates.

Common use cases

• Your app says one due date and the sonographer wrote another, and you want to know which one the hospital will use.
• You have long or irregular cycles, so the period-based date is likely to be off and you want the cycle adjustment applied.
• A 20-week scan measured a week ahead and you want to know whether that is enough to change the due date.
• You want the exact size of the gap in days before asking your midwife about it.

Frequently asked questions