Thank you for your question and congratulations on your wife's
pregnancy. Please note that any information I can provide is no
substitute for direct medical evaluation and advice.
There is only one report in the medical literature that addresses the
question of brain asymmetry detected on ultrasound:
Hering-Hanit R. Achiron R. Lipitz S. Achiron A. Asymmetry of fetal
cerebral hemispheres: in utero ultrasound study. Archives of Disease
in Childhood Fetal & Neonatal Edition. 85(3):F194-6, 2001 Nov.
Here is the abstract:
"BACKGROUND: Slight morphological asymmetry of the cerebral
hemispheres has been observed in fetal and newborn brains. In adults,
sex differences in hemispheric asymmetry have also been reported.
OBJECTIVE: To establish whether cerebral hemisphere asymmetry
correlates with sex in fetuses. METHODS: Left-right cerebral
hemisphere asymmetry, and the correlation with sex, were studied in 51
male and 51 female fetuses of 20-22 weeks gestation, using diagnostic
ultrasound scanning. RESULTS: A total of 102 fetuses were examined.
The diameter of the left hemisphere was larger than that of the right,
in both female and male fetuses. The mean (SEM) diameter of the left
hemisphere was 2.804 (0.174) cm in female fetuses and 2.781 (0.287) cm
in male fetuses; the corresponding values for the right hemisphere
were 2.627 (0.192) cm and 2.681 (0.267) cm. There was no sex related
difference between hemispheric diameters. The interhemispheric
difference was significant for both sexes: male fetuses, p = 0.017;
female fetuses, p = 0.016. CONCLUSIONS: Left-right fetal brain
asymmetry, as measured by in utero ultrasound examination, is apparent
at 20-22 weeks gestation regardless of sex."
This paper examined the differences between the sizes of the brain
hemispheres in male and female fetuses between 20-22 weeks gestation,
which is relevant to the gestational age of your wife's pregnancy.
Although brain asymmetry at this gestational age is well known, the
study did not find a difference in the degree of the asymmetry between
male and female fetuses.
I do wonder if the values you state are correct and if they were
measured correctly. In the study above, the average diameter of the
left hemisphere was found to be 28.04 mm in female fetuses and 27.81
mm in male fetuses. For the right hemisphere females measured 26.27
mm and 26.81 mm. Again, there was a very small asymmetry between left
and right hemispheres in both male and female fetuses, with no
statistical difference between males and females. What concerns me
about the numbers you give is that they are significantly lower than
the diameters quoted in this study. It may be that they were measured
in a nonstandard way or that they are indeed accurate. If this is the
case, then the real question of concern is not asymmetry of the left
and right hemispheres, but the overall size of the fetal brain. The
biparietal diameter (distance across the center of the brain) at 22
weeks should be roughly 57mm (see Table 1 from this paper - free full
text is at this link):
Before getting too concerned, one must determine if the numbers you
give are accurate and if they were measured in a standard fashion. A
repeat ultrasound may be needed to verify the numbers. Inaccurate
measurements can result simply from not being able to get a good view
of the fetus for a variety of reasons.
Small fetal brain (called microcephaly) is an uncommon disorder,
usually the result of a complex genetic abnormality, and is usually
diagnosed around the 28th week. Here are two papers discussing
prenatal microcephaly, which I cite primarily to document how rare the
den Hollander NS. Wessels MW. Los FJ. Ursem NT. Niermeijer MF.
Wladimiroff JW. Congenital microcephaly detected by prenatal
ultrasound: genetic aspects and clinical significance. Ultrasound in
Obstetrics & Gynecology. 15(4):282-7, 2000 Apr.
Dahlgren L. Wilson RD. Prenatally diagnosed microcephaly: a review of
etiologies. Fetal Diagnosis & Therapy. 16(6):323-6, 2001 Nov-Dec.
Both papers are available for a fee online from the above links, but I
would not recommend reading them until you actually have a diagnosis.
Doing so would only serve to cause undo stress and anxiety.
Before making any decisions or performing any invasive tests, the next
step at this point would obviously be to verify the numbers you state
in your question. If they agree with the ones reported by your wife's
ultrasound, then I would recommend a repeat ultrasound. It is more
likely than not that the numbers are inaccurate based on the first
study above and the rarity of microcephaly.
I hope this information was helpful. Please feel free to ask for clarification.