Gestational Age and EDD Calculator
Accurate calculation of pregnancy term and estimated due date.
Gestational Age Calculation: Clinical Guidelines
Accurate determination of gestational age and the estimated due date (EDD) is the cornerstone of obstetric care. Precise calculation optimizes the scheduling of screening exams, evaluates fetal growth, and prevents complications related to prematurity or post-term pregnancy.
Calculation Methods
Two main methods are used and recommended by the American College of Obstetricians and Gynecologists (ACOG):
- Naegele's Rule (LMP): Based on the first day of the last menstrual period. The classic formula adds 9 months and 7 days. Our calculator automatically adjusts this calculation if the patient's cycle is shorter or longer than the standard 28 days.
- Early Ultrasound (CRL): Measuring the Crown-Rump Length (CRL) in the first trimester (ideally between 8 and 13 weeks) is the most accurate method for dating a pregnancy. If the ultrasound EDD differs from the LMP EDD by more than 5-7 days, the ultrasound date should be used.
Term Definitions
A full-term pregnancy generally lasts 280 days (40 Weeks). Clinical definitions are: Preterm (Before 37 Weeks), Early term (37 Weeks to 38 Weeks + 6 d), Full term (39 Weeks to 40 Weeks + 6 d), Late term (41 Weeks to 41 Weeks + 6 d), and Post-term (≥ 42 Weeks).
Pregnancy Trimesters
| Trimester | Period (Weeks) | Key Development |
|---|---|---|
| First | 0 - 13 SA + 6j | Organogenesis, Down syndrome screening. |
| Second | 14 SA - 27 SA + 6j | Fetal growth, morphological ultrasound. |
| Third | 28 SA - 40 SA+ | Lung maturation, preparation for birth. |
References:
1. ACOG Committee Opinion No 700. Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-e154.
[ACOG Official]
2. Naegele FC. Erfahrungen und Abhandlungen aus dem Gebiethe der Krankheiten des weiblichen Geschlechtes. 1812.
[NIH Archive]
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💡 Committed to scientific accuracy, if you notice any clinical or technical discrepancy, please let us know.