EDD Calculator
Estimate your EDD Calculator with our free gynecology & pregnancy calculator. See reference ranges, risk factors, and next-step guidance.
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
EDD Calculator
Calculator
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Formula: EDD = LMP + 280 days + (Cycle Length - 28) | From Conception: Conception + 266 days
Worked example — Due Date: December 6, 2026 | Full term: November 22 - December 20 | Early term: November 8
Formula
EDD = LMP + 280 days + (Cycle Length - 28) | From Conception: Conception + 266 days
The Naegele rule adds 280 days (40 weeks) to the first day of the last menstrual period, adjusted for cycle length variations. From a known conception date, 266 days (38 weeks) are added. For IVF, the equivalent LMP is calculated from the transfer date and embryo age, then the standard 280-day formula is applied.
Worked Examples
Example 1: Standard LMP Calculation
Problem:A woman with a 28-day cycle had her last period on March 1, 2026. What is her estimated due date?
Solution:Using Naegele rule: LMP: March 1, 2026 Subtract 3 months: December 1, 2025 Add 1 year and 7 days: December 8, 2026 Alternatively: March 1 + 280 days = December 6, 2026 Cycle adjustment: 28 - 28 = 0 days Final EDD: December 6, 2026
Result:Due Date: December 6, 2026 | Full term: November 22 - December 20 | Early term: November 8
Example 2: IVF Day 5 Blastocyst Transfer
Problem:A patient had a day 5 blastocyst transfer on March 20, 2026. What is her estimated due date?
Solution:Transfer date: March 20, 2026 Embryo age: 5 days (blastocyst) Equivalent LMP = March 20 - 5 - 14 = March 1, 2026 EDD = March 1 + 280 = December 6, 2026 Alternatively: March 20 + 261 = December 6, 2026
Result:Due Date: December 6, 2026 | Most accurate dating method | Full term begins November 22
Frequently Asked Questions
How is the estimated due date calculated using the Naegele rule?
The Naegele rule is the standard method for calculating the estimated due date (EDD). It adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). An alternative way to apply the rule is to subtract three months from the LMP date, then add one year and seven days. For example, if the LMP was March 1, subtracting three months gives December 1, adding one year and seven days gives December 8 of the following year. The rule assumes a 28-day menstrual cycle with ovulation occurring on day 14. For women with cycles longer or shorter than 28 days, the due date is adjusted by adding or subtracting the difference. Despite its simplicity, only about 4-5% of babies are actually born on their exact due date.
What does estimated due date really mean and how accurate is it?
The estimated due date (EDD) represents the date at which the pregnancy reaches 40 weeks of gestational age, which is the statistical average for pregnancy duration. However, normal full-term pregnancy ranges from 37 to 42 weeks, meaning the EDD is really the midpoint of a five-week delivery window. Studies show that only 4-5% of women deliver on their exact due date, while approximately 80% deliver within 10 days of it. First-time mothers tend to deliver slightly later than the EDD on average, while women who have had previous pregnancies may deliver slightly earlier. The accuracy of the EDD depends heavily on the dating method used, with first-trimester ultrasound being the most precise method, accurate to within plus or minus 5-7 days.
How does cycle length affect the due date calculation?
Cycle length directly affects due date calculation because it determines when ovulation occurred relative to the last menstrual period. The standard Naegele rule assumes a 28-day cycle with ovulation on day 14. Women with longer cycles ovulate later and should have their due date adjusted forward. For example, a woman with a 35-day cycle ovulates approximately 7 days later than average, so her due date should be pushed forward by 7 days (280 + 7 = 287 days from LMP). Conversely, a woman with a 24-day cycle ovulates earlier, and her due date would be 4 days sooner (280 - 4 = 276 days from LMP). This adjustment is important because without it, women with long cycles would be incorrectly classified as post-term, potentially leading to unnecessary interventions.
What is the difference between early term, full term, late term, and post-term?
In 2013, the American College of Obstetricians and Gynecologists redefined term pregnancy into distinct categories based on outcomes data. Early term spans 37 weeks 0 days through 38 weeks 6 days. Full term covers 39 weeks 0 days through 40 weeks 6 days, representing the period with the lowest risk of neonatal complications. Late term encompasses 41 weeks 0 days through 41 weeks 6 days. Post-term begins at 42 weeks 0 days and beyond, carrying increased risks including placental insufficiency, meconium aspiration, and stillbirth. This classification replaced the previous practice of calling anything from 37-42 weeks simply as term. The distinction matters because babies born at 37-38 weeks have higher rates of respiratory problems, feeding difficulties, and NICU admission compared to those born at 39-40 weeks.
How is the due date calculated for IVF pregnancies?
IVF pregnancies offer the most precise pregnancy dating because the exact timing of fertilization and embryo transfer is known. For day 5 blastocyst transfers, the due date is calculated by subtracting 19 days from the transfer date (to determine the equivalent LMP date) and then adding 280 days. Alternatively, 261 days (280 - 19) can be added directly to the transfer date. For day 3 embryo transfers, subtract 17 days from the transfer date to get the equivalent LMP, then add 280 days. The calculation accounts for the fact that in a natural cycle, fertilization occurs approximately 14 days after LMP, and the embryo would naturally reach the blastocyst stage 5 days later. IVF due dates are generally considered the most accurate of all dating methods and should not be changed based on ultrasound measurements.
When should first-trimester ultrasound dating be used instead of LMP dating?
First-trimester ultrasound dating should take priority over LMP dating in several common situations. When the LMP date is uncertain or unknown, ultrasound provides the only reliable dating method. When there is a discrepancy of more than 7 days between the LMP-based due date and ultrasound-based due date in the first trimester, the ultrasound date should be used. Women with irregular menstrual cycles benefit greatly from ultrasound dating, as their LMP-based calculation may be significantly inaccurate. Women who were using hormonal contraception shortly before conception may have unreliable LMP dates. First-trimester ultrasound dating using crown-rump length is accurate to plus or minus 5-7 days, making it more precise than LMP-based calculations for most women. ACOG recommends that all women receive a dating ultrasound in the first trimester when possible.
What prenatal milestones and appointments should I expect based on my due date?
Once the due date is established, a schedule of prenatal milestones unfolds. The first prenatal visit typically occurs at 8-10 weeks, including blood work and dating ultrasound. Nuchal translucency screening occurs at 11-13 weeks. Cell-free DNA testing (NIPT) can be performed from 10 weeks onward. Maternal serum screening (quad screen) is offered at 15-20 weeks. The detailed anatomy ultrasound is performed at 18-22 weeks. Glucose tolerance testing for gestational diabetes screening happens at 24-28 weeks. Rh antibody screening for Rh-negative mothers occurs at 28 weeks. Group B Streptococcus screening is performed at 35-37 weeks. From 36 weeks, prenatal visits increase to weekly frequency. Non-stress testing or biophysical profiles may begin at 36-40 weeks depending on risk factors.
How does the due date affect decisions about labor induction?
The due date plays a central role in decisions about labor induction timing. Elective induction before 39 weeks is generally not recommended unless there is a medical indication, as babies born in the early term period (37-38 weeks) have higher complication rates. The ARRIVE trial demonstrated that induction at 39 weeks in low-risk nulliparous women does not increase cesarean rates and may actually decrease them. For post-term pregnancies (beyond 42 weeks), induction is strongly recommended due to significantly increased risks of stillbirth, meconium aspiration, and macrosomia. Between 41 and 42 weeks, providers typically offer induction with enhanced fetal surveillance. Medical conditions such as gestational diabetes, preeclampsia, or growth restriction may warrant earlier induction regardless of the due date.
Can the due date change during pregnancy?
The official due date may be revised under specific circumstances, though frequent changes should be avoided as they can cause confusion. The most common reason for revision is when a first-trimester ultrasound shows a gestational age that differs from the LMP-based calculation by more than 7 days. In the second trimester, a discrepancy of more than 10-14 days may prompt revision. After 22 weeks, ultrasound dating becomes too imprecise to change the due date. ACOG recommends that once a due date is established by early ultrasound, it should not be changed based on later measurements, as size variations in later pregnancy reflect individual fetal growth patterns rather than dating errors. Consistently using one well-established due date is important for appropriate timing of screening tests, assessments of fetal growth, and delivery planning.
What factors influence whether the baby will arrive before or after the due date?
Multiple factors influence the timing of spontaneous labor onset relative to the due date. First-time mothers deliver on average 3-5 days after the due date, while women with previous deliveries tend to deliver closer to or slightly before the due date. Genetic factors play a role, with studies showing that delivery timing has a hereditary component. Cervical length measured by ultrasound at 18-24 weeks is predictive, with shorter cervixes associated with earlier delivery. Maternal age, body mass index, and race/ethnicity influence delivery timing. Medical conditions including preeclampsia, gestational diabetes, and multiple gestations frequently result in earlier delivery either spontaneously or by induction. The sex of the baby may also play a minor role, with male fetuses associated with slightly longer gestations on average. Psychological stress and physical activity levels have modest effects on delivery timing.
References
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist · Editorial policy
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