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Evidence-Based Pregnancy Dating

Due Date Calculator

Calculate your estimated pregnancy due date and see how far along you are.

Enter the first day of your last menstrual period.

Quick fill:
28 days

Most cycles are 28 days. Adjust if yours is shorter or longer (20–45 days).

20 days (Short)28 days (Typical)45 days (Long)

How the Due Date Is Calculated

Human pregnancy averages 280 days (40 weeks) from the first day of the last menstrual period (LMP), or 266 days (38 weeks) from the date of conception. Several standard clinical methods are used to establish an estimated due date:

1. Last Menstrual Period (Naegele's Rule)

Based on the standard 28-day cycle, Naegele's rule adds 1 year, subtracts 3 months, and adds 7 days to the first day of your LMP (+280 days). When cycle lengths differ, each day above or below 28 days is added or subtracted.

2. Known Conception or Ovulation Date

When the exact date of ovulation or fertilization is known, the estimated due date is calculated by adding 266 days (38 completed weeks) directly to that date.

3. IVF & Frozen Embryo Transfer (FET)

Assisted reproduction provides exact developmental timing. A Day 5 blastocyst transfer adds 261 days to the transfer date; a Day 3 embryo transfer adds 263 days.

4. Ultrasound Biometry (Crown-Rump Length)

Early first-trimester ultrasound scans directly measure fetal length (CRL). If ultrasound dating differs by more than 5–7 days from LMP, clinical guidelines recommend using the ultrasound date.

How Accurate Is a Due Date?

An Estimated Due Date (EDD) is a helpful clinical baseline rather than an exact delivery deadline. In clinical practice, only a small proportion of babies (approximately 4% to 5%) are born on their exact predicted date.

Most spontaneous deliveries occur within a 5-week window spanning from 37 weeks to 42 weeks of gestation. Factors influencing delivery timing include:

  • Maternal and paternal genetics: Family trends often correlate with gestation length.
  • Parity: First-time mothers (nulliparous) statistically tend to deliver slightly later than individuals who have had previous births.
  • Cycle variation: Natural fluctuations in follicular phase length can shift actual ovulation by several days.
  • Individual fetal maturation rates: Biological readiness of the fetal lungs and hormonal signaling trigger spontaneous labor.

How Pregnancy Is Classified Around the Due Date

The American College of Obstetricians and Gynecologists (ACOG) and Society for Maternal-Fetal Medicine (SMFM) categorize the weeks surrounding the estimated due date into four distinct clinical term classifications:

Term ClassificationGestational TimingClinical Overview
Early Term37w 0d – 38w 6dBaby organs are well-formed, though continuing lung and brain development benefits from reaching 39 weeks.
Full Term39w 0d – 40w 6dThe optimal window for newborn respiratory, metabolic, and neurological outcomes. Includes the 40-week due date.
Late Term41w 0d – 41w 6dPregnancy extends past 41 weeks. Providers typically monitor amniotic fluid and fetal well-being more closely.
Postterm42w 0d and beyondPregnancy extending beyond 42 completed weeks. Labor induction is commonly recommended to ensure maternal and fetal safety.

Using a Reverse Due Date Calculator

If you already know your estimated due date, a reverse due date calculator estimates the probable calendar dates when you conceived and your likely fertile window.

Estimated Conception Date

Calculated by subtracting 266 days (38 weeks) from your due date.

Approximate Fertile Window

Spans roughly 5 days prior to ovulation through 1 day after ovulation, reflecting the natural lifespan of sperm and ovum.

* Note: Because ovulation and sperm longevity can vary, a reverse calculation provides an informative estimate rather than absolute biological certainty.

Complementary Tool

Ovulation & Fertile Days Calculator

If you are planning conception or tracking your menstrual cycles, estimate your likely fertile window, upcoming ovulation date, and key testing milestones.

Calculate Fertile Days →

Frequently Asked Questions

How does a due date calculator work?
A pregnancy due date calculator estimates your expected date of delivery (EDD) by adding the standard length of human gestation to a known biological starting point. For calculations based on your last menstrual period (LMP), it adds 280 days (40 weeks) according to Naegele's rule. For known conception or ovulation dates, it adds 266 days (38 weeks). For IVF/FET procedures, it calculates from the exact embryo age and transfer date.
How do I calculate my due date from my last period?
To calculate your due date using your last menstrual period, identify the first day of your last normal period (LMP), add 280 days (40 completed weeks), and adjust for your menstrual cycle length if it is shorter or longer than the standard 28 days. The formula is: Estimated Due Date = LMP + 280 days + (Cycle Length - 28 days).
What is an LMP due date calculator?
An LMP (Last Menstrual Period) due date calculator is the standard clinical method used by healthcare providers when a pregnancy is first confirmed. Because most individuals know the calendar date their last period began better than the exact day of ovulation or fertilization, clinical pregnancy dating measures gestational age starting from the first day of the last menstrual period.
How accurate is a pregnancy due date calculator?
Online calculators provide an estimated baseline benchmark. While based on validated clinical formulas, only a small proportion (around 4% to 5%) of babies are born on their exact estimated due date. Most healthy, full-term spontaneous deliveries occur across a window between 37 weeks and 42 weeks of gestation.
Can I calculate my due date from my conception date?
Yes. If you tracked ovulation with LH test strips, basal body temperature, or know the date of intercourse leading to conception, you can calculate your due date by adding 266 days (38 weeks) directly to your conception date. Human fetal development from fertilization to birth averages approximately 266 days.
How does an IVF due date calculator work?
An IVF due date calculator uses the exact embryo age and transfer date rather than menstrual period estimates. For a Day 3 embryo transfer, 263 days (266 - 3) are added to the transfer date. For a Day 5 blastocyst transfer, 261 days (266 - 5) are added. For a Day 6 blastocyst transfer, 260 days (266 - 6) are added.
How is an FET due date calculated?
A Frozen Embryo Transfer (FET) due date is calculated identically to a fresh embryo transfer based on the developmental age of the embryo at cryopreservation. For example, a frozen Day 5 blastocyst transfer calculates the estimated due date as the transfer date plus 261 days.
Can an ultrasound change my due date?
Yes. If a first-trimester dating ultrasound scan measures fetal crown-rump length (CRL) that differs from your menstrual LMP estimate by more than 5 to 7 days, your obstetrician or midwife may officially adjust your clinical estimated due date to match the ultrasound measurements, which are considered the most accurate dating benchmark in early pregnancy.
How do I use a reverse due date calculator?
A reverse due date calculator works backward from your known or confirmed estimated due date to determine the approximate calendar dates when conception and the fertile window most likely occurred. It subtracts 266 days from the due date to estimate conception and 280 days to estimate the reference LMP date.
When did I conceive based on my due date?
Based on an average 40-week gestation, conception typically occurs approximately 266 days (38 weeks) before your estimated due date, or about two weeks after your reference LMP date. Because sperm can survive for up to 5 days in the reproductive tract and ovulation timing varies, this calculation provides an estimated window rather than an absolute date.
How far along am I based on my due date?
You can determine your gestational age by subtracting 280 days from your estimated due date to find your baseline LMP date, then counting the elapsed calendar weeks and days from that date to today. For example, if 95 days have passed since your reference LMP, you are 13 weeks and 4 days pregnant.
Why isn't my due date the exact day my baby will be born?
A due date is an estimated central midpoint of a normal bell curve rather than an exact deadline. Natural biological variations in follicular phase length, individual fetal growth rates, placental maturity, and maternal physiology all influence when spontaneous labor begins.
What happens if I have an irregular menstrual cycle?
If your menstrual cycle is irregular, calculations based purely on a standard 28-day LMP formula may be less reliable. Adjusting the cycle length input in our calculator helps calibrate the estimate. In clinical practice, healthcare providers rely primarily on early first-trimester ultrasound biometric measurements for individuals with irregular cycles.
What does 40 weeks pregnant mean?
40 weeks pregnant represents 280 days measured from the first day of your last menstrual period. Because conception typically occurs around week 2 (day 14) of a 28-day cycle, the actual developing embryo/fetus is approximately 38 weeks old when you reach 40 completed gestational weeks.
What is the difference between estimated due date and conception date?
The estimated due date (EDD) is the projected calendar date when your pregnancy reaches 40 completed weeks (280 days from LMP). The conception date is the estimated day fertilization occurred (typically 38 weeks / 266 days before the due date). Clinical obstetric medicine measures pregnancy from LMP rather than conception date.
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Health Fitness Calculator Team

Medical Disclaimer

This calculator provides an estimated due date for educational purposes and does not replace professional medical advice. Your healthcare provider may establish or adjust your pregnancy dating based on your clinical history, ultrasound biometry, and individual health factors.