How to use the Pregnancy Calculator

Quick start

1. Open the Pregnancy Calculator.
2. Enter the first day of the last menstrual period, not the last day bleeding occurred.
3. Use the first example, "LMP Apr 1: 28-day cycle", if you want to see a filled-out calculation before entering your own values.
4. Calculate, read the formula line, then copy the result only after the units and assumptions look right.

Best uses

Use this guide when one of these tasks matches what you are trying to do.

• Estimate an expected due date from LMP.
• Check gestational age today.
• Estimate conception timing from cycle length.
• Use a simple date reference before clinical dating is confirmed.

What this calculator is for

The Pregnancy Calculator uses the first day of the last menstrual period and cycle length to estimate due date, gestational age today, conception timing, and trimester.

Use it when you want to: Estimate an expected due date from LMP. Check gestational age today.

What to enter

Good answers start with clean inputs. Before calculating, check the labels, units, and dates so the tool is solving the same problem you actually have.

• Enter the first day of the last menstrual period, not the last day bleeding occurred.
• Adjust cycle length if your usual cycle is shorter or longer than 28 days.
• Use the current date on the page to read gestational age today.

Example walkthrough

Try the calculator example: LMP Apr 1: 28-day cycle. The example result is Estimated due date.

• For an LMP of Apr 1 with a 28-day cycle, the calculator adds about 280 days for the due date.
• If the cycle is longer, ovulation is estimated later and the due date can shift later.

Formula and steps

The calculator starts with the first day of the last menstrual period, adds about 280 days, and adjusts for cycle length.

The formula line on the calculator page is there so the answer is not a mystery. Read it when you need to understand where the number came from.

How to read the answer

Pregnancy dating can change after ultrasound or clinician review. Treat calendar dates as planning estimates.

• Gestational age is counted from LMP, so it is usually about two weeks more than conception age.
• Trimester labels are planning references and may change after clinical dating.

Common mistakes to avoid

Most bad calculator results come from a small input mistake or from using a good estimate for the wrong decision.

• Do not treat the estimated conception date as proof of one exact day.
• Do not use calendar dating instead of ultrasound or clinician guidance.
• Do not forget to adjust for a cycle that is not 28 days.

What to try next

A related calculator can help check the same topic from another angle instead of relying on one number.

• Use Due Date Calculator for a focused due-date page.
• Use Pregnancy Weight Gain Calculator for BMI-based gain references.

Sources and safety notes

This guide uses public-health, clinical, or peer-reviewed references where the calculator needs a specific formula or interpretation boundary.

Source links are provided for transparency, but they do not turn the calculator into medical advice or a replacement for professional care.

• Johns Hopkins Medicine: Calculating a due date
• Johns Hopkins Medicine: Fertile window basics

Examples from the calculator

Common questions

Related tools

• Due Date Calculator Estimate pregnancy due date from LMP and cycle length.
• Pregnancy Conception Calculator Estimate conception date from an expected due date.
• Ovulation Calculator Estimate ovulation date and fertile window from cycle details.

History, privacy, and copying

Recent answers stay visible in the page while you work. The history is kept only in the current browser tab and is not sent to a server.

Copy answer copies the expression and result so you can paste it into notes, homework, a message, or another document.