What is the first day of my last menstrual period (LMP)?

The first day of your last menstrual period (LMP) is the first day you started bleeding, not spotting. This is the standard starting point doctors use to calculate gestational age and your estimated due date because most women know this date more accurately than their conception date.

How accurate is a pregnancy due date calculator?

A pregnancy due date calculator provides an estimate, not a guaranteed delivery date. Only about 5% of babies are born exactly on their due date. The calculation assumes a regular 28-day cycle and a 40-week pregnancy. Your healthcare provider may adjust your due date after an early ultrasound, especially if your cycles are irregular.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1 through 13, when major organs begin forming. The second trimester covers weeks 14 through 27, often bringing renewed energy and visible baby bump growth. The third trimester runs from week 28 until delivery, when the baby gains weight rapidly and prepares for birth.

How do I track my pregnancy week by week?

Our pregnancy calculator shows your current week and day of pregnancy after you enter your LMP. It also displays fetal development milestones for upcoming weeks, helping you understand what is happening at each stage of your pregnancy journey.

Back to Calculators

Pregnancy Due Date Calculator

Use our free pregnancy calculator to instantly find your estimated due date, conception date, current gestational age, and trimester — all from the first day of your last menstrual period (LMP). Whether you conceived naturally or via IVF, get a full week-by-week pregnancy timeline in seconds.

Your Pregnancy Journey

Track your progress from conception to delivery

Pregnancy Details

Last Menstrual Period (LMP)

Enter the first day of your last menstrual period for an LMP-based due date estimate.

Average Cycle Length (days)

Conception Method

Current Progress

Pregnancy Progress 0%

Weeks

Days

Trimester

What Is a Pregnancy Calculator?

A pregnancy calculator is an online tool that helps you estimate key dates in your pregnancy journey. You enter basic information — usually the first day of your last menstrual period (LMP) — and the calculator does the math for you. Within seconds, you can find out your estimated due date, how many weeks and days along you are, which trimester you are currently in, and a rough timeline of important milestones.

How Does a Pregnancy Date Calculator Work?

Most pregnancy date calculators use a method called Naegele's Rule. The simple formula is: Due Date = First Day of Last Period + 280 days (40 weeks). The average pregnancy lasts around 40 weeks, counting from the first day of your last menstrual period — not from conception. This is why your healthcare provider asks about your LMP at your very first appointment. If you do not remember your LMP, some calculators also let you estimate based on conception date, IVF transfer date, or ultrasound measurements.

Pregnancy Calculator Week by Week

One of the most useful features of a week-by-week pregnancy calculator is that it does not just give you a due date and leave you hanging. It maps out your entire pregnancy in stages. The first trimester (weeks 1–12) is when your baby's major organs and structures begin forming. The second trimester (weeks 13–26) is often called the honeymoon phase — many women feel more energized, nausea fades, and you will start showing. The third trimester (weeks 27–40) is the home stretch, when your baby gains weight rapidly and you prepare for labor and delivery.

When to Take a Pregnancy Test

Timing matters a lot when taking a pregnancy test. Most home pregnancy tests detect the hCG hormone in your urine, and that hormone only shows up in detectable levels after a fertilized egg has implanted — which typically happens 6 to 12 days after ovulation. The general rule of thumb is to wait until the first day of your missed period for the most reliable result. Some sensitive tests can detect pregnancy 3 to 4 days before your missed period. Testing too early often leads to false negatives because hCG levels are still too low.

Tips for Getting the Most Out of Your Pregnancy Calculator

Use the first day of your LMP — not the last day, not the middle. The first day is the starting point.
Know your average cycle length — the standard is 28 days, but yours may be different.
Do not stress over the exact date — a due date is a range, not a deadline.
Track week by week — revisit your calculator regularly to stay informed about your baby's development.
Confirm with your doctor — always follow up with a healthcare professional for personalized guidance.

Disclaimer: This calculator and article are for informational purposes only and are not a substitute for professional medical advice. Always consult your healthcare provider with questions about your pregnancy.

Frequently Asked Questions

Enter the first day of your last menstrual period (LMP) into our pregnancy calculator. The tool uses Naegele's Rule, adding 280 days (40 weeks) to your LMP to estimate your due date. You can also adjust for your average cycle length if it differs from 28 days.