Quick start
- Open the Pregnancy Calculator.
- Enter the first day of the last menstrual period, not the last day bleeding occurred.
- Use the first example, "LMP Apr 1, 2026: 28-day cycle", if you want to see a filled-out calculation before entering your own values.
- Calculate, read the formula line, then copy the result only after the units and assumptions look right.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- Estimate an expected due date from the first day of the last menstrual period.
- Check pregnancy week and gestational age today.
- Estimate conception timing from cycle length.
- Use a simple planning date 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, pregnancy week, gestational age today, conception timing, and trimester.
Use it when you want to: Estimate an expected due date from the first day of the last menstrual period. Check pregnancy week and 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.
- Enter your usual cycle length: the number of days from one period start to the next. Keep 28 only if that is close for you.
- Use the current date on the page to read gestational age today.
Example walkthrough
Try the calculator example: LMP Apr 1, 2026: 28-day cycle. The example result is Due Jan 6, 2027; conception around Apr 15.
- For an LMP of Apr 1, 2026 with a 28-day cycle, the calculator estimates a due date of Jan 6, 2027 and conception around Apr 15, 2026.
- For an LMP of Mar 20, 2026 with a 32-day cycle, the estimate shifts to Dec 29, 2026 because ovulation is estimated later than the 28-day default.
Formula and steps
In plain language: The calculator uses Naegele-style dating: due date = first day of LMP + 280 days + (cycle length - 28 days). It estimates ovulation or conception near LMP + cycle length - 14 days and counts gestational age from LMP to today. Read the result together with the notes on the page, because health and fitness numbers often need personal context.
Read the formula note when you need to understand where the number came from, especially before comparing results over time.
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.
- The due date, conception date, and trimester labels are planning references. An early ultrasound or clinician review can update the official date.
Common mistakes to avoid
Most bad results come from a small input mistake or from using a rough estimate for a decision it cannot safely answer.
- 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 enter a last day of bleeding, an uncertain LMP, or a random bleeding date as if it were a confirmed period start.
- Do not forget to adjust for a cycle that is not 28 days, especially if cycles are long, short, or irregular.
What to try next
A related health tool can help check the same topic from another angle, but one number should not replace proper care.
- Use Due Date Calculator for a focused due-date page.
- Use Pregnancy Conception Calculator when you already have a due date and want a backward estimate.
- 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.
Worked examples for Pregnancy Calculator
Due Jan 6, 2027; conception around Apr 15
Due Dec 29, 2026; conception around Apr 7
Due Jan 13, 2027; conception around Apr 22
FAQ in plain language
When should I use the Pregnancy Calculator?
Use it for simple educational checks, trend tracking, or planning tasks like these: Estimate an expected due date from the first day of the last menstrual period. Check pregnancy week and gestational age today. It can help you understand a number, but it cannot explain your whole health situation.
What do the main Pregnancy Calculator inputs mean?
Enter the first day of the last menstrual period, not the last day bleeding occurred. Cycle length means the usual number of days from one period start to the next; use 28 only if that is close for you. If the LMP is uncertain, cycles are irregular, bleeding may not have been a true period, or a clinician has already dated the pregnancy, use the clinician or ultrasound date instead.
What is the Pregnancy Calculator doing with my inputs?
In plain language: The calculator uses Naegele-style dating: due date = first day of LMP + 280 days + (cycle length - 28 days). It estimates ovulation or conception near LMP + cycle length - 14 days and counts gestational age from LMP to today. Read the result together with the notes on the page, because health and fitness numbers often need personal context.
How should I read the Pregnancy Calculator result?
Read the due date as an estimated delivery date from calendar math, not a guarantee of when birth will happen. Gestational age is counted from LMP, so it is usually about two weeks more than conception age. The conception and trimester lines are planning references, and an early ultrasound or clinician review can update the official date.
Why does pregnancy dating start before conception?
Pregnancy dating usually counts gestational age from the first day of the last menstrual period. That means the gestational-age number is often about two weeks ahead of the estimated conception age.
What if my cycle is not 28 days?
The calculator shifts the due date by the difference from 28 days. A 32-day cycle moves the estimate about four days later, while a 26-day cycle moves it about two days earlier. Irregular cycles make calendar dating less reliable.
Can this tell the exact conception date or biological parent?
No. The conception line is an estimate near ovulation, not proof of an exact day, intercourse date, or parentage. Fertilization timing, sperm survival, ovulation shifts, and dating uncertainty all matter.
Related tools
- Due Date Calculator Estimate pregnancy due date from LMP and cycle length.
- Pregnancy Conception Calculator Estimate conception date, window, and LMP from a due date.
- Ovulation Calculator Estimate ovulation date and fertile window from cycle details.
Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
- Health & Fitness Browse the full category for related tools that help with the same job.
- All free tools Search the complete Access Free Tools library by task, category, or tool name.
- All calculator and utility guides Find more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resources Start here when you are not sure which calculator page fits.
Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.