pregnancy period calculator

Use the first day of your last menstrual period (LMP) to estimate your due date and current pregnancy week.

First day of last period (LMP)

Average cycle length (days)

Typical range is 21 to 40 days. If unsure, keep 28.

This calculator provides estimates. Ultrasound dating and your prenatal provider's guidance are more accurate for clinical decisions.

What is a pregnancy period calculator?

A pregnancy period calculator is a simple tool that estimates how far along a pregnancy is based on menstrual cycle information. In most cases, it starts with the first day of your last menstrual period (LMP), then uses standard obstetric timing to estimate your expected due date (EDD), gestational age in weeks and days, and trimester.

It is called a “period calculator” because the reference point is your period, not the day of conception. This is how pregnancy dating is commonly done in medical practice.

How this calculator works

Core dating method

The calculator uses the classic obstetric model:

Pregnancy length is estimated as 280 days (40 weeks) from the first day of LMP.
If your cycle is longer or shorter than 28 days, due date is adjusted by that difference.
Estimated conception date is approximated as about 14 days after LMP, then adjusted by cycle length.

Why cycle length matters

Ovulation does not occur on the same day for everyone. People with longer cycles often ovulate later; people with shorter cycles may ovulate earlier. A cycle-length adjustment can improve estimate quality, especially before an early dating ultrasound is available.

How to use this pregnancy calculator correctly

Enter the first day of your last period, not the last day.
Use your average cycle length over several months if possible.
Click Calculate to view due date, gestational age, and trimester.
If your periods are very irregular, rely more on ultrasound and clinician evaluation.

Understanding your results

Estimated due date (EDD)

Your EDD is an estimate, not a guarantee. Only a small percentage of births happen exactly on the due date. Most healthy deliveries occur in a window around this date.

Gestational age

Gestational age is reported in weeks + days, measured from LMP. For example, “12 weeks 3 days” means 87 days since the first day of your last period.

Trimester breakdown

First trimester: 0 weeks to 13 weeks 6 days
Second trimester: 14 weeks to 27 weeks 6 days
Third trimester: 28 weeks onward

Important limits of online calculators

Online tools are useful for planning and awareness, but they do not replace prenatal care. Seek personalized care if:

Your cycles are irregular or unknown
You conceived shortly after stopping hormonal contraception
You have bleeding, pain, or uncertain dates
You have a history of complications or high-risk pregnancy

An early ultrasound, especially in the first trimester, is generally the most accurate way to confirm dating.

Quick FAQ

Can I be pregnant if my period dates are uncertain?

Yes. If dates are uncertain, use the calculator for a rough estimate and confirm timing with your provider.

Why does my due date change after ultrasound?

Due dates are sometimes updated because fetal measurements can provide a more precise dating method than cycle-based estimation alone.

What if my cycle is not 28 days?

That is common. Entering your average cycle length helps adjust the estimate more realistically.

Medical disclaimer: This content is educational and not medical advice. Always consult a qualified healthcare professional for diagnosis, treatment, and prenatal care decisions.