Skip to main content

Pregnancy Calculator

Estimate your Pregnancy using LMP and Naegele's rule. Enter your last period date to see estimated due date, conception window, and trimester milestones.

Skip to calculator
Medicine & Health

Pregnancy Due Date Calculator

Calculate your pregnancy due date using last menstrual period (LMP). Find gestational age, trimester, and key pregnancy milestones using Naegele's Rule.

Last updated: January 2026Reviewed by NovaCalculator Medical Editorial Team

Calculator

Adjust values & calculate
Understand the Math

Formula

Due Date = LMP + 280 days (adjusted for cycle length)

Based on Naegele's Rule. The standard assumes a 28-day cycle. For each day your cycle is longer than 28, add one day to the due date. Ovulation typically occurs 14 days before the next period.

Last reviewed: January 2026

Worked Examples

Example 1: LMP January 15, 2024 — standard 28-day cycle

LMP: January 15, 2024 | Cycle: 28 days
Solution:
EDD = Jan 15 + 280 days = October 21, 2024
Result: Due Date: October 21, 2024

Example 2: LMP January 15, 2024 — longer 35-day cycle

LMP: January 15, 2024 | Cycle: 35 days (7 days longer than standard)
Solution:
EDD = Oct 21 + 7 days = October 28, 2024
Result: Due Date: October 28, 2024 (shifted 7 days later)
Expert Insights

Background & Theory

The Pregnancy Due Date Calculator is grounded in the established principles and formulas described below. Date and time calculations underpin a vast range of applications from financial settlement to scheduling and age verification. The complexity arises because civil timekeeping uses irregular units: months have 28, 29, 30, or 31 days; years have 365 or 366 days; hours, minutes, and seconds use base-60 arithmetic; and time zones introduce offsets ranging from -12:00 to +14:00 relative to UTC. The Gregorian calendar's leap year rule is a compound condition: a year is a leap year if it is divisible by 4, except for century years, which must be divisible by 400. Thus 1900 was not a leap year but 2000 was. This rule keeps the calendar synchronized with the solar year to within about 26 seconds per year. For algorithmic date calculations, the Julian Day Number provides a continuous integer count of days since January 1, 4713 BCE, eliminating the irregularity of calendar months and making interval arithmetic straightforward. The Unix epoch, by contrast, counts seconds since 00:00:00 UTC on January 1, 1970, and is the basis of POSIX time used in most computing systems. ISO 8601 standardizes date and time representation as YYYY-MM-DD and combined datetime as YYYY-MM-DDTHH:MM:SS±HH:MM, ensuring unambiguous machine-readable interchange across locales that would otherwise differ in day/month/year ordering. Business day calculation requires excluding weekends and, optionally, a jurisdiction-specific list of public holidays. Duration calculations expressed in years, months, and days must account for the variable length of months, making them non-commutative: the interval from January 31 to February 28 is different from the interval from February 28 to March 31. Age calculation algorithms must handle the edge case of birthdays on February 29 and ensure that a person born on December 31 is not counted as one year older on January 1 of the following year until the clock passes midnight. Zeller's Congruence provides a closed-form formula to determine the day of the week for any Gregorian or Julian calendar date using only integer arithmetic.

History

The Pregnancy Due Date Calculator builds on a long history of ideas and practice, traced below. The need to track time and predict astronomical events gave rise to calendrical systems independently across many civilizations. The Babylonians, around 2000 BCE, developed a lunisolar calendar with 12 months of alternating 29 and 30 days, inserting an intercalary month periodically to keep pace with the solar year. They also divided the day into 24 hours and the hour into 60 minutes, a sexagesimal convention that persists in every modern clock. The Egyptian civil calendar used 12 months of exactly 30 days plus five epagomenal days, totaling 365 days. Though simple for administrative purposes, it drifted against the solar year by one day every four years. Julius Caesar, advised by the Egyptian astronomer Sosigenes, reformed the Roman calendar in 45 BCE. The Julian calendar introduced a 365-day year with a leap day every four years, a system that served Europe for over sixteen centuries. By the 16th century, the accumulated error of the Julian calendar had shifted the spring equinox ten days from its ecclesiastically mandated date, disrupting the calculation of Easter. Pope Gregory XIII commissioned the calendar reform that bears his name, and the Gregorian calendar was introduced in Catholic countries in October 1582. The transition required skipping ten days: October 4 was followed by October 15. Protestant and Orthodox countries adopted the reform slowly; Britain and its colonies switched in 1752, Russia not until 1918, and Greece in 1923. The expansion of railways in the 1840s created an urgent practical problem: each city operated on its own local solar time, making train timetables impossible to coordinate. British railways adopted Greenwich Mean Time as a standard in 1847. The International Meridian Conference of 1884 in Washington formalized the prime meridian at Greenwich and established the global framework of 24 time zones. Daylight saving time was first adopted nationally during World War I to reduce coal consumption. The development of atomic clocks after World War II led to the definition of Coordinated Universal Time (UTC) in 1960, accurate to nanoseconds. The Y2K problem of 1999-2000 demonstrated that two-digit year storage in legacy systems could cause widespread failures, prompting a global remediation effort costing an estimated 300 to 600 billion dollars.

Key Features

  • Calculate the exact difference between any two dates expressed in days, weeks, months, and years simultaneously, accounting for leap years and varying month lengths.
  • Add or subtract any combination of years, months, weeks, and days from a starting date to determine a precise future or past date, with results shown in a full calendar format.
  • Compute a person's exact age from their birthdate in years, months, and days as of today or any specified reference date, suitable for legal, medical, and personal use.
  • Count business days between two dates by excluding weekends and optionally filtering out public holidays from a configurable set of regional holiday calendars.
  • Display a live countdown to any target date and time showing the remaining years, months, days, hours, minutes, and seconds, updating in real time.
  • Convert a specific date and time between any two IANA time zones, correctly handling daylight saving time transitions and historical offset changes.
  • Determine the day of the week for any historical or future date using the proleptic Gregorian calendar, supporting dates ranging from antiquity through far-future years.
  • Format a calculated duration in ISO 8601 interval notation as well as plain human-readable text such as '2 years, 4 months, and 11 days' for use in documentation and APIs.
Explore More

Frequently Asked Questions

Pregnancy Due Date Calculator uses Naegele's Rule: Estimated Due Date (EDD) = First day of your last menstrual period (LMP) + 280 days (40 weeks). The formula assumes a standard 28-day cycle with ovulation on day 14. For example, an LMP of January 15, 2024 gives an EDD of October 21, 2024 (280 days later). If your cycle differs from 28 days, the calculator adjusts the due date accordingly.
Only about 5% of babies are born on their exact due date. The EDD based on LMP is accurate to within roughly ±2 weeks for most pregnancies. An early ultrasound (before 12 weeks) is the most accurate method for confirming gestational age, as it can reduce uncertainty to ±5–7 days. Factors like irregular cycles, late ovulation, and embryo implantation timing all affect accuracy.
The first trimester spans weeks 1–12 and covers early organ development; miscarriage risk drops significantly after week 12. The second trimester covers weeks 13–26 and is often called the 'honeymoon phase' — the baby grows rapidly and most anomaly scans are done around week 20. The third trimester runs from week 27 through week 40 (or beyond), ending at birth. Babies born after week 37 are considered full-term.
Naegele's Rule assumes a 28-day cycle. If your cycle is longer than 28 days, ovulation (and likely conception) occurs later, so the due date shifts forward by the same number of days. For example, a 35-day cycle pushes the EDD 7 days later than the standard calculation. Conversely, a shorter 21-day cycle would move the EDD 7 days earlier. Pregnancy Due Date Calculator automatically applies that adjustment when you enter your average cycle length.

Sources & References

  1. 1Naegele's Rule — ACOG Committee Opinion on Methods for Estimating the Due Date
Educational Note: This calculator is provided for educational and informational purposes. Results are based on the formulas and inputs provided. Always verify important calculations independently. NovaCalculator processes calculator inputs client-side; optional analytics follow visitor consent settings.Reviewed by: NovaCalculator Medical Editorial Team — Reviewed against WHO, NIH, and peer-reviewed clinical sources. Last reviewed: January 2026. © 2024–2026 NovaCalculator.

Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy

Pregnancy Calculator Formula

Due Date = LMP + 280 days (adjusted for cycle length)

Based on Naegele's Rule. The standard assumes a 28-day cycle. For each day your cycle is longer than 28, add one day to the due date. Ovulation typically occurs 14 days before the next period.

Pregnancy Calculator — Worked Examples

Example 1: LMP January 15, 2024 — standard 28-day cycle

Problem:LMP: January 15, 2024 | Cycle: 28 days

Solution:EDD = Jan 15 + 280 days = October 21, 2024

Result:Due Date: October 21, 2024

Example 2: LMP January 15, 2024 — longer 35-day cycle

Problem:LMP: January 15, 2024 | Cycle: 35 days (7 days longer than standard)

Solution:EDD = Oct 21 + 7 days = October 28, 2024

Result:Due Date: October 28, 2024 (shifted 7 days later)

Pregnancy Calculator — Frequently Asked Questions

What are the three trimesters of pregnancy?

The first trimester spans weeks 1–12 and covers early organ development; miscarriage risk drops significantly after week 12. The second trimester covers weeks 13–26 and is often called the 'honeymoon phase' — the baby grows rapidly and most anomaly scans are done around week 20. The third trimester runs from week 27 through week 40 (or beyond), ending at birth. Babies born after week 37 are considered full-term.

Pregnancy Calculator — Background & Theory

The Pregnancy Due Date Calculator is grounded in the established principles and formulas described below. Pediatric health monitoring relies on population-based reference standards and validated calculation methods designed for the unique physiology of developing children. Growth percentile charts allow clinicians and parents to interpret a child's weight, height, and head circumference relative to a reference population of the same age and sex. The CDC growth charts, released in 2000, are based on nationally representative survey data from the United States, while the WHO Child Growth Standards, published in 2006 from the Multicentre Growth Reference Study conducted across six countries, describe optimal growth under standardized conditions and are recommended for children under age two. Gestational age calculation following Naegele's rule estimates the expected delivery date by adding 280 days, or 40 weeks, to the first day of the last menstrual period, then subtracting three months and adding seven days. This rule, attributed to Franz Karl Naegele in the early 19th century, assumes a regular 28-day cycle with ovulation at day 14. Ultrasound-based gestational dating, particularly crown-rump length measurement in the first trimester, improves accuracy for cycles with irregular timing. Infant feeding calculations include estimated caloric requirements of 80 to 120 kilocalories per kilogram per day for newborns, and formula volume guidelines of approximately 150 to 200 milliliters per kilogram per day. Breastfed infants typically feed 8 to 12 times in 24 hours with intake estimated by pre- and post-feed weigh-ins when indicated. Pediatric drug dosing is weight-based, expressed in milligrams per kilogram, because body composition, renal clearance, and metabolic enzyme activity differ substantially from adults. Childhood immunization schedules are developed by advisory committees such as the ACIP in the United States and align with WHO immunization recommendations, scheduling vaccines to coincide with periods of maximum immunological response and minimum passive immunity from maternal antibodies. Developmental milestone tracking uses age-normed criteria across motor, language, cognitive, and social domains to identify children who may benefit from early intervention.

History of the Pregnancy Calculator

The Pregnancy Due Date Calculator builds on a long history of ideas and practice, traced below. Pediatrics as a recognized medical discipline has roots in the 17th century, when Thomas Sydenham began distinguishing childhood illnesses from adult diseases, documenting scarlet fever, measles, and whooping cough as distinct conditions with characteristic progressions. However, high infant mortality rates remained a defining feature of pre-industrial societies, with as many as one in three children dying before the age of five in European cities of the 18th century. The decline of infant mortality through the 19th and early 20th centuries came from multiple converging advances: clean water infrastructure and sewage systems reduced enteric disease, Pasteur's germ theory enabled targeted infection control, and the development of pasteurized milk supplies cut infant diarrheal deaths dramatically. Abraham Jacobi, often called the father of American pediatrics, established the first pediatric clinic in the United States in 1860 and advocated for dedicated pediatric hospitals and medical training. The early 20th century saw the institutionalization of well-child care. Stuart Cravioto and Harold Stuart developed early pediatric growth charts in the 1940s using longitudinal data. In 1946, Dr. Benjamin Spock published Baby and Child Care, the best-selling non-fiction book in American history after the Bible, which democratized child health guidance and shifted parenting culture toward responsiveness and individualized care. The book sold over 50 million copies and was translated into 39 languages. The WHO Multicentre Growth Reference Study, conducted between 1997 and 2003 across Brazil, Ghana, India, Norway, Oman, and the United States, produced the 2006 Child Growth Standards based on children raised under optimal conditions with breastfeeding as the norm, setting an international benchmark independent of affluence or ethnicity. Evidence-based parenting research expanded substantially through the late 20th century, producing validated instruments for developmental screening such as the Ages and Stages Questionnaire, and systematic reviews on attachment, sleep, and early language acquisition that now inform clinical and public health guidance globally.

References