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Macrocephaly/Megalencephaly (Increased Head Circumference)

What is this and what is its significance?

A larger than average head circumference for the week of pregnancy (more than one week's difference) is a relatively common finding and has no medical significance if the fetus's head circumference is in the normal range, especially if one of the parents has a greater than average head circumference (measured using a standard tape measure). Even if the fetus's head circumference is above the upper limit of the normal range, this is not exceptional or rare. However, it must be established that the rate of growth of the head is constant, that there are no additional defects, and that there is no enlargement of the lateral ventricles of the brain - in such situations one of the parents almost always also has a larger head circumference.

An increased head circumference (above the normal range), or macrocephaly, is defined as a head circumference two standard deviations greater than the mean circumference for that week of pregnancy (without an increase in the size of the cerebral ventricles). It occurs in approximately 3% of the normal population and is not usually associated with specific medical problems, especially if one of the parents also has a large head circumference and this parent is healthy and normal. It is known that the head circumference is determined genetically and that there are many families whose members have large head circumferences that are not associated with any other problems.

Rarely an increased head circumference may be associated with developmental delay and/or other problems such as epilepsy.

What should be done when macrocephaly is diagnosed?

  • A 3rd level ultrasound scan should be performed to look for additional defects, intracranial tumors, etc., and also to determine the fetus's weight and to measure the width of the cerebral ventricles. In cases where the cerebral ventricles are also dilated (expanded), the situation is different and the significance of this and the possible presence of hydrocephalus as well should be evaluated. In cases where all the body dimensions of the fetus are large, the increased head circumference is less significant, and the cause of the increased body dimensions should be investigated.

  • The head circumferences of the parents and of the other children in the family should be measured.

  • If neither parent has a large head circumference (or one has, and this is accompanied by neurological problems), then it is worth considering genetic counseling. This will enable the parents to be given a full explanation about the significance of the condition and the possibility that the fetus may have a genetic condition, and the advisability of performing amniocentesis in cases where there is considerable enlargement of the fetal head circumference can be discussed.

A combination of two signs: short femur and slightly increased head circumference

The anomaly discussed in this section progresses throughout the pregnancy.

If on ultrasound examination during the first or second trimester the head circumference is found to be greater and the length of the femur less than the number of the weeks of gestation, and the difference between them is one and a half weeks or more, then this requires further investigation to ensure that the difference does not increase, as if it does, this could indicate a problem of bone development.

The vast majority of the cases where a difference has been found are not associated with significant medical problems in the fetus, although such a combination may indicate that the fetus has a bone disorder known as achondroplasia.

Achondroplasia is the most common form of very short stature (the term "dwarfism" is not used nowadays). In this disorder, the shortening of the femur and the enlargement of the head circumference become obvious only in the late stages of pregnancy (week 34 and later), which is relatively late to undertake investigations and to act in accordance with the results. A molecular test can be performed on amniotic fluid cells in order to identify the mutation in the gene responsible for achondroplasia as soon as the combination described above is found, immediately after the system ultrasound scan, while the deviation from the normal is still small and non-diagnostic (in approximately 99% of cases this test is found to be normal). However, these cases should be monitored by performing regular ultrasound examinations, and if the difference increases, amniocentesis can be performed.
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