3D Baby - Imaging by Ultrasound: Screening For Fetal Defects by Dr. Aubrey Milunsky

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Imaging by Ultrasound: Screening For Fetal Defects by Dr. Aubrey Milunsky

Beaming sound waves into a womb containing the fetus and the surrounding areas with contrasting amniotic fluid lets us see remarkably clear images of the fetus.

Diagnostic pulsed ultrasound is safe in pregnancy for both fetus and mother. Three common types of ultrasound techniques are in use, one routine and the second targeted (also called level II or high-resolution) that enables sharper and more precise images of internal fetal organs including the brain, the heart, and abdominal organs. The third type, called Doppler ultrasound, allows evaluation of blood flow through the umbilical cord and placenta.

Ultrasound (or sonar) studies can be done in any of the three trimesters of pregnancy. Routine ultrasound studies in the first trimester help obstetricians precisely determine, within about five days, fetal age through multiple measurements including the distance between the top of the head and the rear end (the crown - rump length). Measurements taken during this time provide the most accurate assessment of fetal age, as long as development is normal. Twins, triplets, etc. are detectable at this time, as is the observation that in 20 – 40 percent of very early twin pregnancies, one twin literally disappears. Fetal death is often detected at this time where no fetal heartbeat is observed.

Measurements taken during the second trimester assess fetal growth, as well as development of key organs including brain, heart, limbs, bones and kidneys. Concerns about fetal growth may emerge during this trimester and can be determined more accurately by serial ultrasound studies at 14, 17 and 20 weeks of pregnancy. Realization that intrauterine growth restriction is present so early in pregnancy raises serious issues including the possibility of a chromosome abnormality. In such instances, amniocentesis using ultrasound guidance of the needle is usually recommended to assess the fetal chromosome status.

The placenta and the volume of amniotic fluid are observed in the second and third trimesters. The amniotic fluid is largely fetal urine, and if there is too small a volume of amniotic fluid (called oligohydramnios), the inference is that something has interfered with fetal urine output – for example, an obstruction at the bladder neck or at some other site from the kidneys on down. More likely, however, is interference with fluid intake by the fetus. Examples include a poorly functioning placenta, a fetal genetic disorder, or a fetus that is “sick” or developmentally abnormal. Fetal growth restriction observed as early as the second trimester, especially with associated oligohydramnios, suggests that a poor outcome to pregnancy is likely. Fetal death, birth defects, mental retardation and handicaps, are all more likely in such instances.

Excess amniotic fluid volume, polyhydramnios, is also not a favorable sign, and just as for oligohydramnios, is much more likely to be seen in the last three months of pregnancy. Birth defects occur more frequently in association with polyhydramnios. Problems in fetal swallowing of amniotic fluid, due for example to obstruction at any site from mouth to anus, or the brain control of swallowing, may result in polyhydramnios. Excess production or problems in processing the amniotic fluid may occur when mothers are diabetic. Serial ultrasound studies allow monitoring of amniotic fluid volume as one index of fetal health.

High-resolution ultrasound is usually recommended when risks of fetal defects are above normal. Such studies focus on the integrity of heart structure and the anatomy of the brain. In addition, remarkable pictures of the limbs can be obtained, including the digits. This is sometimes of importance for certain syndromes where there might be six fingers and six toes, for example. Kidney abnormalities and bladder problems can also be seen. Defects of the spine including spina bifida can be determined by high-resolution analysis and defects of the long bones make a diagnosis of hereditary bone diseases possible as early as the second trimester. Determination of fetal sex is usually more than 95 percent accurate.

On occasion, a tiny area of increased echo-density is seen in the fetal abdomen, raising an important question concerning the possibility of cystic fibrosis. Specific tests via amniocentesis may be needed to exclude this possibility. However, since routine cystic fibrosis DNA carrier tests should have been offered either before pregnancy or in the earliest weeks of pregnancy, parents should already have been recognized as carriers and at risk, and already have been offered amniocentesis for specific DNA studies if both were carriers.

Echo-dense spots may also be seen in the bowel or the heart in disorders where there is a chromosome abnormality in the fetus. While the risk in those circumstances may only be 1 or 2 percent, amniocentesis for prenatal chromosome studies is usually recommended. Noting that the nasal bone (the developing nose) is not fully developed or absent in fetal Down syndrome is proving to be an important and useful ultrasound observation.

Finally, all of the studies mentioned may be frustrated by an inability to obtain clear images. This is most frequently the case in the face of massive maternal obesity, or when there is a dramatic reduction in the amount of surrounding amniotic fluid. On occasion, clear images are not easy to obtain because of the awkward position of the fetus.

All in all, ultrasound examination of the fetus in the first and second trimester has proved to be extraordinarily valuable in screening as well as diagnosing of specific fetal defects.

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