Baby Due Date Calculator - Free Online Tool

Calculate your estimated due date using multiple medically recognized formulas with trimester breakdown, cultural features, and prenatal screening dates

Welcome to our free Baby Due Date Calculator, one of the most comprehensive online due date tools available. Whether you are trying to find out when your little one will arrive, comparing the results of different calculation formulas, or just curious about your baby's predicted zodiac sign and birthstone, this tool has everything you need in one place. Understanding your estimated due date (EDD) is one of the first and most important steps in any pregnancy journey. Your EDD helps you and your healthcare team plan prenatal care visits, schedule important screenings and tests, monitor fetal development week by week, and mentally and practically prepare for the arrival of your baby. While only about 4 to 5 percent of babies actually arrive on their exact due date, most healthy pregnancies deliver within a window of one to two weeks on either side of the EDD. This calculator supports five distinct calculation methods: Last Menstrual Period (LMP), Conception Date, IVF Transfer Date, Ultrasound Dating, and Ovulation Date. Each method reflects a different starting point for measuring pregnancy, and your healthcare provider may use one or more of these approaches depending on your medical history and available information. The LMP method is the most common. It uses Naegele's Rule, which adds 280 days to the first day of your last menstrual period. This formula assumes a standard 28-day cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, our cycle length adjustment automatically shifts your estimated due date to account for the difference. A 35-day cycle, for example, adds 7 extra days to the standard 280. One of the features that makes this calculator unique is its side-by-side formula comparison. In addition to the standard Naegele's Rule, we calculate your due date using two alternative formulas: Mittendorf-Williams and Parikh's Formula. The Mittendorf-Williams rule, developed from a 1990 study by Mittendorf and colleagues, uses a different duration depending on whether you are expecting your first baby (nulliparous, 288 days from LMP) or a subsequent baby (multiparous, 283 days from LMP). Studies have found this formula to be approximately twice as accurate as Naegele's Rule in some populations. Parikh's Formula, designed specifically for women with non-standard cycle lengths, calculates the EDD as LMP plus 9 months minus 21 days plus your average cycle length. For women who conceived through IVF, the calculator supports Day 3, Day 5, and Day 6 embryo transfer options. The math accounts for the age of the embryo at transfer: a Day 5 blastocyst transfer, for example, means the embryo is already 5 days old, so only 261 days remain rather than the full 266. Ultrasound dating uses the gestational age measured during an ultrasound scan combined with the scan date to work backward to an estimated LMP and forward to a due date. First-trimester ultrasounds are considered the most accurate dating method when they differ significantly from LMP-based calculations. Beyond the due date itself, this tool provides an in-depth pregnancy profile including your current gestational age in weeks and days, which trimester you are in, how many days and weeks remain until your due date, the safe delivery window from 37 to 42 weeks, and term classification (preterm, early term, full term, late term, or post-term) for any given date. The trimester breakdown shows you the exact start and end dates for all three trimesters with key developmental milestones for each. A visual pregnancy progress donut chart shows how far along you are as a percentage of the full 280-day journey, while a trimester bar chart gives you a horizontal visual of your position within each trimester. For fun, we also calculate your baby's predicted Western zodiac sign, monthly birthstone, and Chinese zodiac animal based on the estimated due date. These cultural features are entirely for enjoyment and reference, not medical guidance. The prenatal screening schedule provides dates for key appointments and tests based on your estimated due date, including your first prenatal visit, nuchal translucency scan, anatomy scan, glucose tolerance test, Group B Strep test, and weekly checkups near the end of pregnancy. All calculations are performed entirely in your browser. No personal data is stored or transmitted to any server. This calculator is designed for informational purposes only and does not replace the advice of your healthcare provider. Always consult your doctor or midwife for personalized medical guidance.

Understanding Baby Due Date Calculation

A typical pregnancy lasts approximately 280 days (40 weeks) from the first day of the last menstrual period, or about 266 days (38 weeks) from conception. Due dates are statistical estimates based on population averages, and individual pregnancies vary considerably.

Naegele's Rule — The Standard Formula

Naegele's Rule is the globally accepted standard for calculating estimated due dates. It adds 280 days to the first day of the last menstrual period, assuming ovulation occurs on day 14 of a 28-day cycle. The formula can be remembered as: add 1 year, subtract 3 months, and add 7 days to the LMP date. When your cycle differs from 28 days, the formula is adjusted by adding or subtracting the difference. A woman with a 35-day cycle would add 7 extra days, while a woman with a 21-day cycle would subtract 7 days. Despite being developed in the 1800s, Naegele's Rule remains the most widely used and clinically accepted method worldwide.

Mittendorf-Williams and Parikh's Formulas

The Mittendorf-Williams rule, published in a 1990 study, challenged the one-size-fits-all approach of Naegele's Rule by finding that first-time mothers (nulliparous) tend to deliver at 288 days from LMP, while mothers who have given birth before (multiparous) tend to deliver at 283 days from LMP. Studies have found Mittendorf-Williams to be approximately twice as accurate as Naegele's Rule in certain populations. Parikh's Formula was designed specifically for women with irregular or non-28-day cycles: EDD equals LMP plus 9 months minus 21 days plus average cycle length. This is mathematically equivalent to LMP plus 259 days plus cycle length, which means a woman with a 35-day cycle gets EDD equals LMP plus 294 days.

IVF, Ultrasound, and Ovulation Methods

IVF pregnancies use the most precise due date calculations because the exact timing of fertilization is known. Day 3 embryos add 263 days from transfer, Day 5 blastocysts add 261 days, and Day 6 blastocysts add 260 days. Ultrasound dating uses the measured gestational age at the time of the scan combined with the scan date to calculate the expected due date. First-trimester crown-rump length (CRL) measurements are accurate to within 5 to 7 days. The ovulation date method treats the ovulation date as equivalent to the conception date, adding 266 days — the full fetal gestation period from fertilization.

What the Due Date Really Means

The estimated due date is not a deadline — it is the midpoint of a normal delivery window. Fewer than 5 percent of babies are born on their exact due date. About 60 percent of births occur within one week of the EDD, and about 90 percent within two weeks. A normal pregnancy is considered full term between 39 and 40 weeks and 6 days. Early term is 37 to 38 weeks and 6 days, late term is 41 weeks to 41 weeks and 6 days, and post-term is 42 weeks or beyond. Most healthcare providers begin considering intervention if a pregnancy goes significantly past 41 to 42 weeks.

Due Date Formulas

Naegele's Rule (LMP)

EDD = LMP + 280 days

The standard formula used worldwide. Assumes a 28-day cycle with ovulation on day 14. For non-standard cycles, add or subtract the difference from 28 days.

Gestational Age

Gestational Age = Today − LMP

The number of days since the first day of the last menstrual period, expressed in weeks and days. Full term is 40 weeks (280 days).

Mittendorf-Williams Rule

EDD = LMP + 288 days (first pregnancy) or LMP + 283 days (subsequent)

An alternative formula from a 1990 study that differentiates between first-time mothers (nulliparous) and those who have given birth before (multiparous).

Trimester Boundaries

T1: Weeks 1–12 | T2: Weeks 13–26 | T3: Weeks 27–40

Pregnancy is divided into three trimesters. The first trimester covers organ formation, the second covers rapid growth, and the third covers final maturation and birth preparation.

Pregnancy Reference Tables

Pregnancy Milestones by Week

Key developmental milestones throughout pregnancy from early embryonic stages to full term delivery.

Week	Milestone	Clinical Significance
Week 6	First heartbeat detectable	Confirms viability via Doppler ultrasound
Week 12	End of first trimester	Miscarriage risk drops significantly
Week 16–18	Quickening (first movements felt)	Mother begins to feel fetal movement
Week 20	Anatomy scan	Detailed structural survey of all organs
Week 24	Viability milestone	Survival outside the womb becomes possible with NICU care
Week 37	Early term begins	Baby is considered fully formed
Week 39–40	Full term	Lowest risk delivery window
Week 42	Post-term	Induction typically recommended

Trimester Overview

Summary of each trimester including duration, key developments, and common symptoms.

Trimester	Weeks	Key Developments	Common Symptoms
First	1–12	Organ formation, heartbeat, embryo to fetus	Nausea, fatigue, breast tenderness
Second	13–26	Rapid growth, movement felt, sex identifiable	Energy returns, visible bump, backache
Third	27–40	Fat accumulation, lung maturation, birth prep	Swelling, Braxton Hicks, difficulty sleeping

Worked Examples

Calculate Due Date from LMP (March 15)

A woman's last menstrual period began on March 15, 2026. She has a standard 28-day cycle and this is her first pregnancy.

Apply Naegele's Rule: EDD = March 15 + 280 days

Count forward 280 days from March 15: March has 16 remaining days + April (30) + May (31) + June (30) + July (31) + August (31) + September (30) + October (31) + November (30) = 260 days through November 30. 280 − 260 = 20 more days into December.

EDD = December 20, 2026

Mittendorf-Williams (first pregnancy): March 15 + 288 days = December 28, 2026

Parikh's Formula: March 15 + 259 + 28 = March 15 + 287 = December 27, 2026

The estimated due date is December 20, 2026 by Naegele's Rule. The Mittendorf-Williams and Parikh formulas suggest December 27–28, giving a realistic delivery window of late December 2026.

Determine Current Gestational Age

A woman's LMP was January 5, 2026, and today's date is March 10, 2026.

Calculate days elapsed: January has 26 remaining days + February (28) + March 10 = 64 days

Convert to weeks and days: 64 ÷ 7 = 9 weeks and 1 day

Determine trimester: Week 9 falls in the first trimester (weeks 1–12)

Calculate days remaining: 280 − 64 = 216 days remaining

Progress percentage: (64 ÷ 280) × 100 = 22.9% complete

The gestational age is 9 weeks and 1 day. The pregnancy is 22.9% complete, in the first trimester, with 216 days (approximately 30.9 weeks) remaining until the EDD of October 12, 2026.

How to Use the Baby Due Date Calculator

Choose Your Calculation Method

Select the method that matches your situation from the five available options: Last Menstrual Period (LMP), Conception Date, IVF Transfer Date, Ultrasound Dating, or Ovulation Date. If you are unsure, the LMP method is the most common and is used by most healthcare providers as the starting point for pregnancy dating. The LMP tab automatically appears when you open the calculator.

Enter Your Date and Adjust Settings

Enter the relevant date for your chosen method. For LMP, also enter your average cycle length if it differs from the standard 28 days — this adjusts all three formula results automatically. For the LMP and Mittendorf-Williams formula, select whether this is your first pregnancy or a subsequent one, as the Mittendorf-Williams formula uses different day counts for each. For IVF, select whether your embryo was a Day 3, Day 5, or Day 6 transfer. For ultrasound, enter the scan date and the gestational age in weeks and days shown on your report.

Review Your Due Date and Formula Comparison

Your estimated due date appears as the large hero result at the top of the results panel. Below it you will see your current gestational age, days remaining, and trimester status. Scroll down to see the Formula Comparison section, which shows your due date calculated using Naegele's Rule, Mittendorf-Williams, and Parikh's Formula side by side — giving you a range of estimates from three scientifically recognized methods. This is unique to this calculator and helps you understand the range of likely delivery dates.

Explore Charts, Cultural Features, and Screening Schedule

The pregnancy progress donut chart and trimester timeline bar give you visual snapshots of your pregnancy journey. In the cultural features section, see your baby's predicted Western zodiac sign, monthly birthstone, and Chinese zodiac animal based on the estimated due date. Click the screening schedule toggle to see a personalized table of key prenatal appointments and their estimated dates based on your EDD. Use the print button to save a copy of all your results.

Frequently Asked Questions

What is the difference between Naegele's Rule, Mittendorf-Williams, and Parikh's Formula?

Naegele's Rule is the oldest and most widely used formula, adding 280 days to the first day of your last menstrual period and adjusting for cycle length. The Mittendorf-Williams rule, published in 1990, found that first-time mothers tend to deliver at 288 days from LMP while mothers who have given birth before deliver at around 283 days. Some studies have found it to be roughly twice as accurate as Naegele's Rule. Parikh's Formula was specifically designed for women with irregular cycles: EDD equals LMP plus 9 months minus 21 days plus cycle length. For a 28-day cycle, Parikh's formula gives the same result as Naegele's, but for longer or shorter cycles it provides a different estimate. Seeing all three side by side helps you understand the realistic range of your delivery date.

How do I calculate my due date if I had IVF?

For IVF pregnancies, the calculation uses your embryo transfer date and the age of the embryo at the time of transfer. A Day 3 embryo transfer adds 263 days to the transfer date (because the embryo is already 3 days old out of the full 266-day fetal gestation from conception). A Day 5 blastocyst transfer adds 261 days, and a Day 6 blastocyst adds 260 days. This gives a very precise estimate because the exact timing of fertilization is known in IVF. Select the IVF Transfer Date method in this calculator, enter your transfer date, and choose the appropriate embryo age from the dropdown to get your result.

Why does my doctor sometimes give me a different due date than this calculator?

Healthcare providers may adjust your due date based on first-trimester ultrasound measurements, which are considered the most accurate method for pregnancy dating. If your crown-rump length (CRL) measurement at the ultrasound differs significantly from your LMP-based estimate — typically more than 5 to 7 days in the first trimester — your provider will often use the ultrasound date instead. The ultrasound reflects actual fetal development rather than an estimated fertilization date. It is also possible that your provider uses a slightly different cycle length assumption or a different formula. The due date shown by this calculator should be close to what your provider calculates, and the formula comparison feature shows the range of medically supported estimates.

Is it normal if I deliver before or after my due date?

Yes, absolutely. Only about 4 to 5 percent of babies are born on their exact due date. The estimated due date is a statistical midpoint based on population averages, not a precise prediction for any individual pregnancy. About 60 percent of full-term births occur within one week of the EDD, and about 90 percent occur within two weeks. A normal delivery window spans from 37 to 42 weeks of gestation. Babies born between 39 and 40 weeks and 6 days are considered full term. Between 37 and 38 weeks and 6 days is called early term, 41 weeks is late term, and 42 or more weeks is post-term. Your healthcare provider will monitor you closely in the final weeks and discuss options if your pregnancy goes significantly past your due date.

How accurate are online due date calculators compared to ultrasound dating?

LMP-based calculators like this one are a very good starting estimate, but they assume that your ovulation occurred on day 14 of a 28-day cycle, which is not true for everyone. Women with irregular cycles, polycystic ovary syndrome (PCOS), or cycles that vary significantly in length may find LMP-based results less accurate. First-trimester ultrasound is considered the most accurate dating method, with accuracy within 5 to 7 days. Second-trimester ultrasounds are accurate within about 7 to 14 days. This is why many providers will confirm or adjust the LMP-based estimate using an early ultrasound. Use this calculator as an excellent starting estimate and confirm with your healthcare provider.

What do the Chinese zodiac and Western zodiac results mean?

The zodiac and cultural results shown in this calculator are purely for fun and are based on your baby's estimated due date. The Western zodiac sign is determined by the calendar date of the EDD using the standard astrological date ranges. The monthly birthstone is the traditional gemstone associated with the birth month of the EDD. The Chinese zodiac animal is determined by the Chinese New Year of the year your baby is expected to be born — since Chinese New Year falls between late January and late February, babies born near that window may fall into the zodiac year before or after the standard calendar year. None of these cultural features have medical significance; they are intended as a fun addition to your due date calculation results.