Pregnancy Due Date Calculator

Finding out when your baby is expected to arrive is one of the first and most exciting steps of pregnancy.

Reviewed by: CalcMojo Editorial Team

This pregnancy due date calculator uses Naegele’s rule, the standard clinical method used by obstetricians worldwide, to estimate your Expected Date of Delivery (EDD) based on the first day of your last menstrual period (LMP). The method has been the foundation of obstetric dating since Dr. Franz Karl Naegele published it in 1812 and remains the starting point for prenatal care scheduling today.

The calculation assumes a 28-day menstrual cycle and estimates that pregnancy lasts 280 days (40 weeks) from the first day of the LMP, or 266 days from conception. Enter your LMP date or conception date, and the tool returns your estimated due date along with a trimester-by-trimester timeline, your current gestational age, and key developmental milestones.

It is important to understand that a due date is an estimate, not a deadline. Only about 4% of babies are born on their exact due date. The majority of full-term deliveries occur within a two-week window around the EDD, between 38 and 42 weeks of gestation. Your healthcare provider may adjust your due date based on ultrasound measurements, particularly if your cycle length differs from 28 days or the LMP date is uncertain.

How Naegele’s Rule Works

Naegele’s rule calculates the estimated due date using a simple formula applied to the first day of the last menstrual period:

EDD = LMP + 1 year – 3 months + 7 days

Alternatively expressed: add 280 days (40 weeks) to the first day of the LMP.

For example, if your LMP was January 1, your estimated due date would be October 8.

The formula assumes:

A regular 28-day menstrual cycle
Ovulation occurring on day 14 of the cycle
Conception occurring within 24 hours of ovulation
A gestational period of 280 days from LMP (266 days from conception)

If your menstrual cycle is consistently longer or shorter than 28 days, the due date shifts accordingly. For a 35-day cycle, ovulation typically occurs around day 21 instead of day 14, pushing the due date approximately 7 days later than Naegele’s standard calculation predicts. This calculator allows you to adjust for cycle length variation.

Understanding Gestational Age

Gestational age counts pregnancy from the first day of the last menstrual period, even though conception typically occurs about two weeks later. This means that on the day of conception, you are already considered approximately 2 weeks pregnant by gestational age reckoning. This can be confusing, but it is the standard dating system used universally in obstetrics.

Gestational age is measured in weeks and days from the LMP. A pregnancy described as "12 weeks, 3 days" means 12 weeks and 3 days have passed since the LMP.

Fetal age (also called embryonic age or conceptional age) counts from the actual date of conception and is approximately 2 weeks less than gestational age. Fetal age is used less frequently in clinical practice but may appear in some developmental references.

Full-term pregnancy spans 37 to 42 weeks of gestational age, with the due date at 40 weeks. Deliveries before 37 weeks are considered preterm, and deliveries after 42 weeks are considered post-term.

The Three Trimesters

Pregnancy is divided into three trimesters, each approximately 13-14 weeks long. Understanding these divisions helps you track developmental milestones and anticipate the physiological changes at each stage.

First Trimester (Weeks 1-12). This period encompasses the most rapid phase of embryonic development. By week 5, the heart begins to beat. By week 8, all major organ systems have begun forming. By week 12, the fetus is approximately 2.5 inches long, and the risk of miscarriage drops significantly. Common symptoms during this trimester include nausea (morning sickness), fatigue, breast tenderness, and frequent urination. Most prenatal genetic screening tests are performed during this trimester.

Second Trimester (Weeks 13-26). Often called the most comfortable trimester, as first-trimester nausea typically subsides and the abdomen is not yet large enough to cause significant discomfort. The fetus grows rapidly, with movement (quickening) typically felt between weeks 18 and 22. The anatomy scan ultrasound, usually performed around week 20, assesses fetal development and can often determine sex. By week 24, the fetus reaches viability, meaning survival outside the womb is possible with intensive medical intervention.

Third Trimester (Weeks 27-40). The fetus gains significant weight as fat stores develop for temperature regulation after birth. The lungs mature throughout this trimester, with surfactant production increasing substantially after week 34. Common maternal symptoms include back pain, swelling, difficulty sleeping, and Braxton-Hicks contractions. Regular prenatal visits increase in frequency during this trimester, typically moving to every two weeks and then weekly as the due date approaches.

Due Date Accuracy and Ultrasound Dating

Naegele’s rule provides a useful starting estimate, but its accuracy depends heavily on how well the underlying assumptions match your individual biology.

First-trimester ultrasound. An ultrasound performed between 8 and 13 weeks is the most accurate method of establishing a due date, with a margin of error of approximately plus or minus 5-7 days. If the ultrasound-based due date differs from the LMP-based date by more than 7 days, most practitioners will adjust the due date to match the ultrasound.

Second-trimester ultrasound. Measurements taken between 14 and 20 weeks are accurate to within plus or minus 10-14 days. After 20 weeks, individual growth variation makes ultrasound dating less reliable.

Factors that reduce LMP-based accuracy: irregular menstrual cycles, recent discontinuation of hormonal contraception, breastfeeding at the time of conception, uncertain LMP date, and polycystic ovary syndrome (PCOS) which can cause irregular ovulation.

Studies show that first-trimester ultrasound dating reduces the rate of post-term pregnancy diagnosis and induction by providing a more accurate due date than LMP-based calculation alone. The American College of Obstetricians and Gynecologists (ACOG) recommends that the most accurate dating method available be used to establish the EDD, with first-trimester ultrasound preferred when available.

What Happens Around Your Due Date

Understanding the timeline around your due date helps with preparation and reduces anxiety about timing.

37-38 weeks: Early term. The baby is considered early term. All major organs are functional, though continued time in the womb provides benefits, particularly for brain development and lung maturation.

39-40 weeks: Full term. This is the optimal delivery window. Babies born during this period have the best overall outcomes. The due date falls at exactly 40 weeks.

41 weeks: Late term. If labor has not begun by 41 weeks, your provider will typically discuss options including membrane sweeping (a technique to encourage labor onset) and scheduling induction.

42 weeks: Post-term. Pregnancies extending beyond 42 weeks carry increased risks, including placental deterioration, decreased amniotic fluid, and increased fetal size complicating delivery. Most practitioners will recommend induction by 41-42 weeks, and ACOG suggests considering induction at 39 weeks for certain situations.

The due date should guide your preparation timeline. Most healthcare providers recommend having your hospital bag packed, car seat installed, and birth plan discussed by 36-37 weeks, as early labor can begin at any time from that point forward.

Factors That Affect Due Date

Several factors can cause your actual delivery date to differ from the calculated EDD.

First pregnancies. First-time mothers tend to deliver slightly later than the due date on average. Studies show first pregnancies go approximately 1-3 days past the EDD more frequently than subsequent pregnancies.

Maternal age. Older mothers have a slightly increased likelihood of delivering past the due date.

Ethnicity. Some studies have found differences in average gestational length across ethnic groups, though the clinical significance and appropriate adjustments remain debated.

Multiple gestation. Twins typically deliver around 36-37 weeks and triplets around 32-33 weeks. The standard 40-week due date does not apply to multiple pregnancies.

Medical conditions. Gestational diabetes, preeclampsia, and other pregnancy complications may necessitate earlier delivery than the due date.

Use our Pregnancy Weight Gain Calculator to track healthy weight gain through each trimester based on IOM 2009 guidelines.

This calculator provides general estimates based on published formulas. It is not medical advice and does not replace consultation with a qualified healthcare provider.

Frequently Asked Questions

How is the due date calculated?

The standard method uses Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. Your healthcare provider may adjust the date based on ultrasound measurements, particularly if your cycle is irregular or longer than 28 days.

How accurate is the due date?

Only about 4% of babies are born on their exact due date. Most full-term deliveries occur within a two-week window around the EDD, between 38 and 42 weeks. A first-trimester ultrasound can narrow the estimate to plus or minus 5-7 days. The due date is best viewed as the center of a probable delivery window.

Can my due date change?

Yes. If an ultrasound, particularly in the first trimester, shows that fetal size does not match the LMP-based due date by more than 7 days, your provider will likely adjust the due date. Once established using the most accurate method available, the due date should not be changed based on later ultrasounds, as growth variation increases with gestational age.

What if I do not know my last menstrual period date?

If your LMP date is unknown or unreliable, your provider will use an early ultrasound to establish the due date. First-trimester ultrasound is the most accurate alternative dating method. You can also estimate based on a known conception date by adding 266 days.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of the last menstrual period, making it approximately 2 weeks longer than fetal age. Fetal age (conceptional age) counts from the actual date of conception. Clinical practice universally uses gestational age, so when your provider says you are 12 weeks pregnant, the fetus is approximately 10 weeks old.

When should I see a doctor after getting a positive pregnancy test?

Most providers schedule the first prenatal appointment between 8 and 10 weeks of gestational age. Contact your provider promptly upon receiving a positive test so they can schedule your initial visit and begin any recommended early testing. Seek immediate care if you experience heavy bleeding, severe abdominal pain, or other concerning symptoms.

Is it safe to go past my due date?

Going one to two weeks past the due date is common and usually safe with proper monitoring. Your provider will typically increase surveillance after 40 weeks, often including non-stress tests and amniotic fluid checks. Most guidelines recommend discussing induction between 41 and 42 weeks due to increased risks of post-term pregnancy.

How does cycle length affect the due date?

The standard calculation assumes a 28-day cycle with ovulation on day 14. If your cycle is consistently longer, ovulation occurs later, pushing the actual due date beyond what Naegele’s rule predicts. For a 35-day cycle, add approximately 7 extra days to the standard due date. This calculator allows you to adjust for your specific cycle length.

Sources & Methodology

Due date calculated using Naegele’s rule (Naegele FK, 1812), the standard clinical method for estimating expected date of delivery.
Gestational age conventions from the American College of Obstetricians and Gynecologists (ACOG) Committee Opinion No. 700, "Methods for Estimating the Due Date," 2017.
Delivery date statistics from Jukic AM et al., "Length of human pregnancy and contributors to its natural variation," Human Reproduction, 2013.
Trimester developmental milestones from ACOG patient education materials and the National Institute of Child Health and Human Development (NICHD).
Ultrasound dating accuracy from Butt K, Lim K, "Determination of Gestational Age by Ultrasound," Journal of Obstetrics and Gynaecology Canada, 2014.

Default values shown are illustrative. Always verify with your healthcare provider. Data accurate as of: March 2026