Estimated Due Date - the projected date of delivery, typically 40 weeks from the last menstrual period.
Gestational Age
Pregnancy age counted from the first day of the last menstrual period, about 2 weeks more than actual fetal age.
Trimester
Pregnancy divided into three trimesters: 1st (weeks 1-12), 2nd (weeks 13-27), 3rd (weeks 28-40).
Full Term
A pregnancy that reaches 39-40 weeks. Early term is 37-38 weeks. Post-term is after 42 weeks.
CRL
Crown-Rump Length - the measurement from the top of the head to the bottom of the buttocks, used in first-trimester ultrasound dating.
Nulliparous
A woman who has never given birth. First pregnancies tend to last slightly longer than subsequent ones.
Real-World Examples
Example 1
LMP Calculation with Regular Cycle
Last period: January 15, 2026, Regular 28-day cycle, First pregnancy
Result: Naegele: Oct 22, Mittendorf-Williams: Oct 30 (first pregnancy adds 8 days), Parikh: Oct 22 (28-day cycle matches Naegele).
Example 2
Irregular Cycle Comparison
Last period: Feb 1, 2026, 35-day cycle, Second pregnancy
Result: Naegele: Nov 15, Parikh: Nov 22 (adjusted for 35-day cycle), Mittendorf-Williams: Nov 11 (subsequent pregnancy). Parikh is likely most accurate here.
Understanding Your Due Date
Due Dates Are Estimates
Only about 5% of babies arrive on their exact due date. The normal range for delivery is 37-42 weeks. Think of your due date as the middle of a 5-week window rather than a precise target.
Why Different Methods Give Different Dates
Naegele's Rule assumes all women have 28-day cycles and ovulate on day 14. Mittendorf-Williams accounts for the fact that first pregnancies tend to go longer. Parikh's formula adjusts for your actual cycle length. An early ultrasound measures the embryo directly, making it the most accurate single method.
First Trimester Ultrasound
An ultrasound between 8-12 weeks is the most accurate way to date a pregnancy, accurate to within 3-5 days. If it differs from the LMP date by more than 7 days, the ultrasound date is typically used.
Factors That Affect Timing
First pregnancies tend to go slightly past the due date. Subsequent pregnancies often come a few days earlier. Cycle length matters - longer cycles push the due date later. Genetic factors also play a role.
Frequently Asked Questions
How accurate is a due date calculated from my last period?
An LMP-based due date assumes ovulation on day 14 of a 28-day cycle. If your cycle is irregular or longer/shorter than 28 days, the estimate may be off by several days. A first-trimester ultrasound (8-12 weeks) is accurate to within 3-5 days and is considered the gold standard for dating.
What percentage of babies are born on their due date?
Only about 4-5% of babies arrive on their exact due date. The normal delivery window spans 37-42 weeks of gestation. Most babies arrive within 1-2 weeks of the due date. First-time mothers tend to deliver slightly later than the due date on average.
Can my due date change after an ultrasound?
Yes. If a first-trimester ultrasound differs from the LMP-based date by more than 5-7 days, most providers will adjust the due date to match the ultrasound. Later ultrasounds are less accurate for dating because fetal growth rates vary more in the second and third trimesters.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of the last menstrual period (LMP), which is about 2 weeks before conception. Fetal age (also called embryonic age) counts from the actual date of conception. A pregnancy described as 10 weeks gestational age is approximately 8 weeks fetal age.
Which due date calculation method is most accurate?
A first-trimester ultrasound (8-12 weeks) is the most accurate single method, within 3-5 days. Among formula-based methods, Parikh's formula tends to be most accurate for women with irregular cycles since it accounts for individual cycle length. Mittendorf-Williams is useful for distinguishing first vs. subsequent pregnancies.
When is a pregnancy considered full term?
Full term is 39 weeks 0 days through 40 weeks 6 days. Early term is 37-38 weeks 6 days. Late term is 41 weeks 0 days through 41 weeks 6 days. Post-term is 42 weeks and beyond. Most providers will discuss induction between 39-41 weeks depending on individual circumstances.