perinatology calculator

What is a perinatology calculator?

A perinatology calculator helps estimate important pregnancy milestones, especially in higher-risk or closely monitored pregnancies. In practice, these tools are used to estimate gestational age, expected date of delivery (EDD), and timing of prenatal testing. Accurate dating is central to obstetric care because screening windows, fetal growth interpretation, and delivery planning all depend on gestational age.

What this calculator provides

• LMP-based dating: EDD and gestational age from the first day of your last menstrual period.
• Cycle-length adjustment: Adjusts estimates when cycles are shorter or longer than 28 days.
• Ultrasound-based dating: Calculates EDD from gestational age measured during ultrasound.
• Dating comparison: Shows discrepancy between LMP and ultrasound estimates.
• Suggested working EDD: Highlights whether discrepancy may support redating.

How the calculations work

LMP formula

Traditional obstetric dating assumes pregnancy length is about 280 days (40 weeks) from LMP. This calculator applies a cycle-length correction so that estimated ovulation shifts relative to a standard 28-day cycle.

• Base EDD = LMP + 280 days
• Adjusted EDD = LMP + 280 + (cycle length − 28) days
• Estimated conception date = LMP + (cycle length − 14) days

Ultrasound formula

If an ultrasound measured gestational age on a known date, we can project EDD by subtracting that measured age from 280 days:

• Ultrasound EDD = ultrasound date + (280 − measured GA in days)
• Back-calculated LMP = ultrasound date − measured GA in days

Redating logic

Obstetric guidelines often use trimester-specific discrepancy thresholds to decide whether to keep LMP dating or switch to ultrasound dating. This page uses common practical cutoffs:

• Up to 13w6d: redraw EDD if discrepancy is 5 days or more
• 14w0d to 21w6d: threshold 7 days
• 22w0d to 27w6d: threshold 10 days
• 28w0d and later: threshold 14 days

Why accurate dating matters in perinatology

In maternal-fetal medicine and perinatology, timing can change management. A few days may affect whether a test is considered early, on-time, or late. Examples include first-trimester screening windows, anatomy scan interpretation, growth restriction diagnosis, and planning intervention in medically complicated pregnancies.

How to use this tool responsibly

• Use the earliest high-quality ultrasound available when possible.
• Enter exact values from reports (weeks and days).
• If periods are irregular, place less confidence in LMP-based dating.
• Discuss any discrepancy with your OB/GYN, midwife, or MFM specialist.

Important limitations

This calculator does not diagnose complications, assess fetal well-being, or replace physician judgment. It cannot account for all clinical variables such as uncertain LMP recall, delayed ovulation, assisted reproduction timing, or differing institutional protocols. Always defer to your care team and your official ultrasound report.