Gestational sac diameter (GS)
The gestational sac can be visualized as early as 4.5 weeks. It increases by about 1 mm per day. As the sac is not usually round, an average of the length, width and depth is made. The acccuracy of dating using GS size is low and can be off by a whole week. This is therefore not recommended. Fetal crown-rump length should be used wherever possible for dating a pregnancy. (Table)
Yolk sac diameter (YS)
Before placental circulation is established, the yolk sac is the primary source of exchange between the embryo and the mother. Between 7 and 11 weeks, the YS can reach a diameter of up to 7mm, after which it will decrease in size. Absence of the YS in the presence of an embryo is always abnormal and is assiciated with fetal demise. A larger than normal YS is also associated with adverse outcome in the fetus. In general a YS diameter of greater than 5.6 mm before 10 weeks is likely to be associated with abnormal fetal outcome. (Table)
Head Circumference (HC)
The head circumference is used similar to the BPD for dating. It is supposed to be better than the BPD because it compensates for the shape of the fetal head ( for example a very flat head will give a smaller BPD ). However the measurement itself is technically more difficult to make and carries with it a higher degree of measurement error. It's use is valuable in fetuses with abnormal head shape. Another measurement, the Cephalic Index, has a similar function. It is the ratio between the two axis of the fetal head. A value of under 74% would represent an excessively flat head.
The nuchal skin fold
The nuchal skin fold is increased in cases where skin oedema is present. And this occurs in Turner's syndrome, Down's syndrome and a number of chromosomal abnormalities. Between 10 to 13 weeks, the normal nuchal fold is less than 3 mm and after 16 weeks it should not exceed 6-7 mm.
Other long bone (limb) measurements
Charts for other long bones in the fetal body are all available, such as for the humerus, radius, ulnar, tibia and fibula. These are important mainly in the diagnosis of congenital malformations and not so much as in the assessment of gestational age of the fetus.
Normal values for many other fetal body measurements are available, such as sizes for the cerebellum, heart chambers, ear, kidneys, and lengths of the foot, clavicles or distances between two orbits etc .... These are useful when assessing certain fetal anomalies in which a particular part of the fetus is known to be affected.
Ratios of the Head circumference to the Abdominal circumference (HC/AC ratio) and of the Femur length to Abdominal circumference (FL/AC ratio) are often used to assess fetal growth. As the AC tends to shrink more in fetal growth retardation, the ratios may be helpful in situations when a dating scan is not available. On the other hand, results from such ratios may be misleading as for example the ratio may appear normal in a growth-retarded fetus when both the limbs and the abdomen are affected to such a degree that both are shrunken.
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