Biometry: measurements of fetal somatic structures in an attempt to construct the gestational age.
Biparietal diameter: transverse diameter of the fetal skull from the leading edge (outer edge) of the near field to the leading edge to the far field (inner edge of skull), through the transthallamic plane.
Abdominal circumference: a measure along the outer perimeter of the fetal abdomen in a true transverse plane at the level of the junction of the umbilical vein and the hepatic vein.
Dolichocephaly: abnormally shaped fetal head that is "flatter."
Brachycephaly: abnormally shaped fetal head that is more round in shape.
Cephalic index: the ratio of the biparietal diameter and the occipital frontal diameter. This index allows one to determine whether the fetal head is of normal shape, dolichocephalic, or brachycephalic.
There is likely no more important piece of information used to manage pregnancies than correct gestational age dating. Gestational age allows the practitioner to recommend the correct age-appropriate serum screening, diabetic screening, administration of anti-D immunoglobulin, group B streptococcal testing, and timing of delivery. The use of the patient's last menstrual period (LMP) to establish dating is fraught with imprecision, particularly if the woman has had irregular ovulation, such as from the use of hormonal contraception, lactation, or oligoovulation. A woman's memory of her LMP may also be imprecise. Consequently, the term certain LMP is valid if specific criteria are present (Table 5-1). Even with a certain LMP, the gestational age is more accurately established with sonographic parameters in the first or early second trimester. Careful and meticulous tables that correlate biometric measurements with gestational age are useful.
Table 5-1CRITERIA FOR CERTAIN LMP ||Download (.pdf) Table 5-1 CRITERIA FOR CERTAIN LMP
|History of regular menses |
|Normal interval between menses (22 to 34 days) |
|No use of hormonal therapy (oral contraceptive or other agent) within 3 months of LMP |
|Patient is sure of LMP (has written down date on calendar) |
|No bleeding or spotting since LMP |
|Last LMP was normal in character and length |
IMPORTANCE OF A CRITICAL APPROACH
The literature contains many tables and nomograms that describe the normal growth of various fetal parameters, such as the biparietal diameter, abdominal diameters, femur, and orbits. Some of these tables were established with great care and respect as to basic mathematical principles. Others, however, were prepared in a less careful manner. Deciding which table to use and knowing its limitations are important in everyday practice. For example, using a table whose "confidence limits" were graphically drawn instead of mathematically computed (or worse, a table that does not provide confidence limits) may be imprecise. The basic principles involved are well established, and the software required for analysis is currently available for most microcomputers. Knowing the basic concepts described in this chapter enables an understanding of the seemingly complicated and esoteric language ...