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Key Terms
Biometry: measurements of fetal somatic structures in an attempt to establish the menstrual/gestational age or ensure appropriate interval growth.
Menstrual age: conventional manner of dating a pregnancy based on the first day of the last menstrual period. Menstrual age is synonymous with gestational age and 14 days longer than the conception date. Usually referred to as estimated gestational age (EGA) to infer the inherent lack of precision.
Biparietal diameter: transverse diameter of the fetal skull through the transthalamic plane measured from the leading edge (outer edge of skull) of the near field to the leading edge of the far field (inner edge of skull).
Head circumference: a measure along the outer perimeter of the fetal skull in the transthalamic plane with the cavum septi pellucidi seen anteriorly and the tentorial hiatus seen posteriorly.
Abdominal circumference: a measure along the outer perimeter of the fetal abdominal soft tissue in a true transverse plane at the level of the junction of the umbilical vein and the hepatic vein.
Femur length: measurement of the axial length of the ossified portions of the femur.
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There is likely no more important piece of information used to manage pregnancies than the correct assessment of gestational age. The accurate determination of gestational age underlies all obstetric management decisions, from the recommendation of the correct gestational age appropriate screening or diagnostic genetic tests to the timing of diabetic screening, to the administration of anti-D immunoglobulin, to group B streptococcal testing, and most importantly, to the assessment of fetal viability and well-being and the appropriate timing of delivery.
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Before the development of ultrasound technology, the menstrual age was based on the reported first day of the patient’s last menstrual period (LMP) and confirmed by physical examination of the gravid uterus and postnatal examination of the fetus. The use of the patient’s LMP to establish dating is fraught with imprecision, particularly if the woman does not have a typical 28- to 30-day menstrual cycle, has had irregular menses resulting from lactation or oligoovulation, or became pregnant while on birth control. A woman’s memory of her LMP may also be imprecise, with studies reporting up to 45% of women being unsure of their LMP date.1 Even with a certain LMP, the ovulation to implantation interval may vary by as many as 11 days and tends to occur later than calculated.2 Further complicating identification of the exact timing of fertilization, sperm can survive in the reproductive tract for 5 to 7 days. Understanding the inherent variations from the “normal” menstrual cycle (Table 6-1) explain why the LMP has been shown to consistently underestimate gestational length and potentially lead to nonindicated obstetrical procedures such as induction of labor for postdates.3
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