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Gestational age is a key piece of data used by healthcare providers to determine when to perform various screening tests and assessments of the fetus and mother throughout pregnancy. Gestational age may be assessed at any time during pregnancy, and several modes of assessment exist, each requiring diverse equipment or skills and associated with varying degrees of accuracy. Obtaining more accurate estimates of gestational age through better diagnostic approaches may initiate more prompt medical management of a pregnant patient. This activity describes the many modalities used to correctly estimate gestational age and highlights the role of the interprofessional team in caring for pregnant patients. Objectives: Explain how to assess gestational age with an ultrasound. Describe the non-sonographic methods for estimating gestational age. Explain the clinical significance of correctly estimating gestational age using ultrasound. Explain the importance of enhancing care coordination amongst the interprofessional team to optimize the use of ultrasound for estimation of gestational age. Access free multiple choice questions on this topic.

Gestational age is a key piece of data used by healthcare providers to determine the timing of various screening tests and assessments of the fetus and mother throughout pregnancy. Gestational age may be assessed at any time during pregnancy, and several modes of assessment exist, each requiring different equipment or skills and with varying degrees of accuracy. Obtaining more accurate estimates of gestational age through better diagnostic approaches may initiate more prompt medical management of a pregnant patient. There are numerous approaches to assess gestational age. Before sonography, practitioners had to rely on detailed histories and physical exam findings. In particular, determining the date of the last known menstrual period was of significance. Inputting the date of the last menstrual period into various formulas can give both an estimated gestational age and an estimated delivery date. Physical exam findings such as determining uterine fundal height is also a method for estimating gestational age.[1][2] Ultrasound has emerged as the more accurate method of assessing fetal gestational age, especially in the first trimester. Both transvaginal and transabdominal probe assessments are used to obtain a more accurate measurement of gestational age. Transvaginal is more helpful in first trimester pregnancies. Multiple parameters have been described using ultrasound to aid in calculating gestational age and will be described here. Additionally, postnatal scoring systems involving focused physical and neurologic exams may also provide insight into gestational age.[3][4]

Prenatal Techniques Non-Sonographic Methods for Determining Gestational Age Naegele’s Rule: This rule assumes a standardized 28-day menstrual cycle with fertilization occurring on day 14. However, many women have irregular cycles with variability in the length of the follicular phase affecting ovulation. History of last menstrual period may be confounded by early pregnancy bleeding, hormonal contraceptive use, or incorrect recall of last menstrual period date.[7] Uterine Size: This finding primarily relies on the provider’s physical examination. This may be confounded by maternal factors including but not limited to obesity, multiple gestation, leiomyoma, and fibroids. The mother may also have a retroverted uterus which will alter normal progressive landmarks. Individual practitioner skill and experience make findings less reliable and reproducible. Sonographic Methods for Determining Gestational Age Crown-rump length (CRL): This parameter becomes less accurate for predicting gestational age and delivery date with the advancement of gestation due to normal embryonic development and variability in anatomic positioning. Once the CRL exceeds 84 mm, the BDP serves as a better indicator for gestational age.[8] Biparietal Diameter (BPD): This parameter becomes limited after 22 weeks gestation due to normal biologic development with variations in fetal size and shape. If there is a physiologic or pathologic cause for the skull size and shape to be altered, the BPD may produce false measurements.[9] Head Circumference (HC): This parameter is useful for obtaining gestation age, but multiple landmarks need to be identified before taking measurements. After 22 weeks, there is significant variation due to normal development affecting size and shape.[10] Femur Length (FC): Multiple errors may occur when obtaining this parameter including but not limited to, non-ossified portions of the femur and not visualizing the full femur. This can lead to incorrect gestational age calculations. Average femur lengths may differ among certain ethnic groups or can be indicative of pathology. Again, this parameter does have variability after 22 weeks due to normal biologic development.[10]

Femur Length (FC): Multiple errors may occur when obtaining this parameter including but not limited to, non-ossified portions of the femur and not visualizing the full femur. This can lead to incorrect gestational age calculations. Average femur lengths may differ among certain ethnic groups or can be indicative of pathology. Again, this parameter does have variability after 22 weeks due to normal biologic development.[10] Abdominal Circumference (AC): This parameter has a wide margin of error for determining gestational age due to multiple factors including asymmetry of the abdomen, changes with respiration and movement, and user skills. Like the other parameters, significant variability is seen after 22 weeks.[10] Ossification Centers: These parameters do not directly correlate with exact gestational age. Presence of these ossified centers are rather markers of fetal maturity which is seen later in pregnancy.[11] Postnatal Techniques Dubowitz Method: Due to a large number of criteria needed to perform the exam, this method may be more difficult to perform on sick and preterm infants. Also, it can take up to 20 minutes to perform this exam. Studies have shown that this method tends to overestimate the gestational age.[12] New Ballard Score: Although this exam is quicker to perform, studies have shown that this system can overestimate gestational age in preterm infants.[13]

It is important to obtain a gestational age in all pregnancies to provide the standard of care medical management for both mother and fetus. A combination of history, physical exam, early sonography in the first trimester, and prenatal assessments are all essential to obtaining a more accurate gestational age. Different techniques may be more useful depending on how far along the pregnancy has progressed. Early sonography has been shown to be the most useful predictor of gestational age; however, other late modalities are available to help determine age.