Pregnancy Gestational Age Calculator
Created by: Olivia Harper
Last updated:
Compare gestational age and estimated due date from LMP, conception date, and early ultrasound CRL to support pregnancy dating reconciliation.
Pregnancy Gestational Age Calculator
PregnancyCompare gestational age and estimated due date from LMP, conception date, and early ultrasound CRL for pregnancy dating reconciliation.
Use a first-trimester CRL when available.
What is a Pregnancy Gestational Age Calculator?
A pregnancy gestational age calculator estimates how far along a pregnancy is using common dating methods such as LMP, conception date, and first-trimester ultrasound CRL. Unlike a simple due date calculator, it is built to compare those methods side by side and show where they agree or diverge.
That matters because dating disagreements are common. Some users know their LMP but not ovulation timing. Others have IVF or tracked conception dates. Early ultrasound can add another layer and sometimes becomes the clinically preferred method when the dates do not line up well.
This tool keeps the current gestational age, estimated due date, and discrepancy explanation together so the user can understand why a due date may be revised.
How Pregnancy Dating Reconciliation Works
Each dating method generates a gestational age and an estimated due date. LMP assumes a pregnancy length of about 280 days from the first day of the last menstrual period. A known conception date uses about 266 days from conception. Early CRL uses first-trimester ultrasound dating logic based on fetal size at the scan date.
When multiple methods are provided, the calculator compares them and highlights whether the discrepancy is small or large enough that ultrasound dating would commonly receive greater weight. The result is educational and should not replace the official dating chosen by your maternity team.
Core pregnancy dating rules
LMP due date = LMP + 280 days
Conception-based due date = conception date + 266 days
Early CRL can be converted to gestational age at scan and then projected to an EDD
Ultrasound often takes priority when first-trimester discrepancy from LMP is clinically meaningful
Example Scenarios
Example 1: LMP only
A user with only an LMP can still estimate gestational age and due date, but the calculator keeps the result framed as a planning tool rather than a final dating decision.
Example 2: LMP versus ultrasound
If early ultrasound CRL suggests a noticeably different age than LMP, the calculator shows the discrepancy and explains why the ultrasound date may be preferred.
Example 3: Known conception or IVF timing
A user with a known conception date can compare that method with LMP and ultrasound to better understand why the dates align or drift apart.
How People Use This Calculator
- Compare pregnancy dating methods in one result set.
- Estimate gestational age and due date from LMP, conception, and CRL inputs.
- Understand why early ultrasound may revise an expected due date.
- Support pregnancy dating conversations with a clinician or sonographer.
Tips for Pregnancy Dating Comparison
Use the most reliable known input. If ovulation or transfer timing is uncertain, LMP and ultrasound context may be more useful than a guessed conception date.
Do not treat this as a tool to self-correct official records. Its value is in understanding the comparison logic behind dating reconciliation.
Frequently Asked Questions
How is this different from a due date calculator?
A simple due date calculator usually gives one estimated date from one input. This tool focuses on comparing multiple dating methods and explaining why one method may take priority when there is disagreement.
Why is early ultrasound often prioritized?
Early ultrasound dating is often more reliable than recalled cycle timing, especially if cycles were irregular or ovulation was not clearly known. That is why clinicians may revise a due date when first-trimester measurements disagree enough with LMP dating.
What is CRL?
CRL stands for crown-rump length. It is an early ultrasound measurement often used to estimate gestational age in the first trimester.
Can conception date be more accurate than LMP?
Yes, if conception or transfer timing is truly known. IVF dates and very closely tracked ovulation timing can sometimes provide more precise dating than LMP alone.
Should I change my due date myself based on this calculator?
No. Use the tool to understand the logic behind pregnancy dating reconciliation, then follow the dating chosen by your maternity clinician or ultrasound report.
Sources and References
- ACOG Methods for Estimating the Due Date.
- NICE antenatal care and dating ultrasound guidance.
- Early pregnancy ultrasound dating literature using CRL.
Medical Note
Pregnancy Gestational Age Calculator is for educational planning only. It does not replace obstetric, midwifery, ultrasound, dietetic, or emergency care.