How do you calculate the expected date of delivery (EDD) in pregnancy?

Calculating Expected Date of Delivery (EDD) in Pregnancy

The EDD should be calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP) when the menstrual cycle is regular and 28 days, or by using early ultrasound measurements performed before 24 weeks of gestation, which provides accuracy within 5-14 days. 1

Primary Methods for EDD Calculation

Last Menstrual Period (LMP) Method

• Add 280 days to the first day of the LMP when the patient has a regular 28-day menstrual cycle and accurately recalls the date 1, 2
• Gestational age is reported as the number of weeks and days from the LMP 1
• This method assumes conception occurred on day 14 of the menstrual cycle 1
• For menstrual cycles that differ from 28 days, adjustments must be made - the standard 280-day calculation will be inaccurate 3
• Research suggests that adding 283 days rather than 280 days may reduce systematic bias in prediction, though 280 days remains the standard 4, 5

Ultrasound Dating Method

• Early ultrasound performed before 24 weeks of gestation estimates gestational age within 5-14 days 1
• First trimester ultrasound measurements (crown-rump length) provide the most accurate dating 1
• Second trimester fetal head circumference measurements can predict EDD with a median pregnancy length of approximately 283 days 5
• Ultrasound dating is particularly valuable when LMP is unknown, uncertain, or when menstrual cycles are irregular 3

Assisted Reproductive Technology (ART)

• For ART pregnancies, GA is calculated by adding 14 days to the number of completed weeks since fertilization 1
• Dating is based on clinical factors such as timing of intrauterine insemination or embryo transfer 1
• This method provides highly accurate gestational age assignment 1

Best Obstetric Estimate

• The "best obstetric estimate" combines early ultrasound findings with LMP date to determine the most accurate GA 1
• When both methods are available, early ultrasound (particularly first trimester) typically takes precedence if there is discrepancy 1
• This combined approach accounts for variations in ovulation timing and cycle length 1

Practical Calculation Tools

Electronic vs Manual Calculators

• Electronic gestational age calculators are strongly recommended over manual wheel calculators 6
• Manual GA wheels demonstrate significant inaccuracy with individual errors up to 4 days and systematic bias throughout the year 6
• Only one manual device in a validation study showed no significant bias compared to the 280-day control 6
• Electronic devices consistently showed no error in validation studies 6

Expected Delivery Date (EDD) Coding in Medical Records

• In German claims data, 82.4% of pregnancies had at least one coded EDD during prenatal care 2
• When multiple EDDs are recorded, they typically show concordance with median differences of only 5 days when discordant 2
• Using the most frequently coded EDD provides a reliable estimate when multiple dates exist 2

Common Pitfalls and Caveats

Accuracy Limitations

• Both clinical examination and ultrasound have inherent margins of error - no method predicts the exact delivery date with certainty 1, 7
• Gestational age based on completed weeks creates artificial boundaries (e.g., a fetus at 23 6/7 weeks differs minimally from one at 24 0/7 weeks) 1
• Ultrasound-estimated fetal weight introduces additional uncertainty when incorporated into outcome predictions 1

Calculation Errors

• Midwives and clinicians show both random and systematic errors when calculating EDD manually, particularly when menstrual cycles differ from 28 days 3
• Underestimation of GA from EDD is a common systematic bias that can have medical consequences, especially at early gestations 3
• 72% of clinicians believe their manual calculation method is accurate, despite evidence showing significant errors 6

Special Circumstances

• When LMP is unknown or uncertain, rely exclusively on early ultrasound dating 3
• For irregular menstrual cycles, the standard LMP calculation will be systematically inaccurate 3
• Manual calculation devices should be validated before use by comparing device-calculated EDD with a 280-day control at five points throughout the year 6

Clinical Application

• The first trimester is defined as GA of 13 weeks 6 days or less 1
• Approximately 22.9% of women remain pregnant 7 days after the EDD, and 3.6% remain pregnant 14 days after the EDD 5
• The term "normal first-trimester pregnancy" should be avoided since abnormalities may be undetectable at early GA; instead use "normal first-trimester ultrasound examination" 1