Conception Calculator by Ultrasound

Calculator Inputs

Ultrasound scan date

Gestational weeks at scan

Extra gestational days

Crown rump length, mm

Cycle length, days

Luteal phase, days

Accuracy margin, days

Date format

Example Data Table

Scan Date	Gestational Age	Cycle Length	Estimated Conception	Estimated Due Date
2026-05-01	8 weeks, 3 days	28 days	2026-03-17	2026-12-08
2026-05-01	10 weeks, 0 days	30 days	2026-03-08	2026-11-27
2026-05-01	12 weeks, 2 days	26 days	2026-02-16	2026-11-04

Formula Used

Estimated last period date = ultrasound scan date - gestational age at scan.

Ovulation offset = cycle length - luteal phase.

Estimated conception date = estimated last period date + ovulation offset.

Standard conception date = estimated last period date + 14 days.

Estimated due date = estimated last period date + 280 days.

CRL fallback age = 8.052 × square root of CRL in millimeters + 23.73 days.

Possible conception range = estimated conception date plus or minus the selected accuracy margin.

How to Use This Calculator

Enter the date shown on the ultrasound report.
Enter the gestational age measured on that scan.
Add crown rump length only when report age is missing.
Adjust cycle length and luteal phase when known.
Select an accuracy margin for a wider or tighter range.
Press calculate and review the result above the form.
Download a CSV or PDF copy for your records.

Understanding Ultrasound Based Conception Estimates

Ultrasound dating helps estimate when pregnancy likely began. The scan usually reports gestational age. That age counts from the estimated last period, not from fertilization. This calculator reverses that report. It subtracts the measured age from the scan date. Then it adds the likely ovulation offset.

Why Results Are Ranges

Conception is rarely known to the exact day. Ovulation can shift. Sperm can live for several days. Ultrasound measurements also have normal variation. For that reason, the calculator shows a central date and a possible date range. The accuracy margin lets you choose a cautious window.

Key Inputs

Start with the ultrasound exam date. Enter the gestational age written on the report. Use weeks and days. If you only have crown rump length, enter the millimeters. The tool can estimate age from that value. Cycle length and luteal phase refine the ovulation offset. Default values match a 28 day cycle with ovulation near day 14.

What The Output Means

The estimated last period date anchors the timeline. The conception estimate points to likely ovulation or fertilization timing. The fertile window covers days before and just after that estimate. The due date is a planning date, not a promise. Many births occur before or after it.

Practical Use

Use the result for records, planning, and discussion. Save the CSV for spreadsheet work. Save the PDF for a simple note. Compare the dates with your clinical report. Ask a qualified maternity professional before changing care plans.

Limitations

This page does not diagnose pregnancy age. It cannot confirm paternity. It cannot replace a clinician, sonographer, or prenatal chart. First trimester scans are usually more useful for dating than later scans. Later growth differences can affect age estimates. Irregular cycles and assisted reproduction may also need special handling.

Best Practices

Enter the earliest reliable scan when possible. Check every date carefully. Keep the margin realistic. Review the result with your provider. Use the calculator as an organized estimate, not as final medical advice.

Data Privacy

No entries are stored by this file unless your server logs requests. Downloads are created from the current form values. Clear the page when using a shared device. Avoid sharing private pregnancy details online.

FAQs

How does this calculator estimate conception?

It subtracts the ultrasound gestational age from the scan date. That gives an estimated last period date. It then adds an ovulation offset based on cycle length and luteal phase.

Is the conception date exact?

No. It is an estimate. Ovulation timing, sperm survival, scan timing, and measurement variation can shift the likely date by several days.

What is gestational age?

Gestational age is pregnancy age counted from the estimated first day of the last menstrual period. It is usually about two weeks longer than fetal age.

Why is cycle length included?

Cycle length helps estimate ovulation timing. A longer cycle may shift ovulation later. A shorter cycle may shift it earlier.

Can I use crown rump length only?

Yes. Enter CRL when the report does not show weeks and days. The tool will estimate gestational age from that measurement.

Why does the due date differ from my clinic date?

Your provider may use a preferred scan, clinical history, or assisted reproduction details. Medical records should guide care decisions.

Does this calculator confirm paternity?

No. Date estimates cannot confirm paternity. Only appropriate testing can answer that question reliably.

Should I change appointments based on this result?

No. Use the result for planning and discussion only. Ask a qualified maternity professional before changing care, tests, or appointments.