Pregnancy Dating Tool
Choose the most reliable reference—last menstrual period, early ultrasound, or due date—to calculate gestational age for today or any reference date.
Leave blank to calculate based on today.
How to Use This Calculator
Select the best available dating method
LMP works if cycles are regular. Ultrasound or IVF dating overrides LMP when discrepancies arise.
Enter accurate data
Use the exact dates and gestational ages documented in medical records.
Consult your provider
This tool supports planning but official dating decisions should come from your obstetric care team.
Formula
LMP method: Gestational age = (As-of date − Adjusted LMP)
Ultrasound method: GA = GA at scan + (As-of date − Scan date)
EDD to GA: GA = 280 − (EDD − As-of date)
Equivalent LMP: LMP = EDD − 280 days
Full Description
Accurate gestational age dating enables timely prenatal screening, fetal growth assessment, and delivery planning. Naegele’s rule provides a quick estimate from the LMP, but ultrasound—particularly first trimester crown-rump length—is superior when available. For IVF pregnancies, embryo transfer dates and embryo age offer precise timing. This calculator harmonizes these approaches to present consistent gestational age information.
Frequently Asked Questions
Which method should I use?
Use LMP if cycles are regular and you have accurate dates. Use ultrasound dating if there’s a discrepancy or if cycles are irregular. IVF patients should use transfer data.
Why does my chart show a different gestational age?
Clinicians may adjust the due date based on guidelines. Follow the dating information in your medical record.
Can I change my due date later?
Due dates are rarely changed after the first trimester. Discuss any concerns with your obstetric provider.
Does this calculator replace medical advice?
No. It is an educational tool. Always confirm gestational age and due date with your clinician.