Ultrasound Imaging: Prenatal Testing
Second and Third Trimester Ultrasounds
Fetal Growth: 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 the middle trimester and can be determined more accurately by serial ultrasound studies at 14, 17, and 20 weeks of pregnancy.
Chromosomal Abnormalities: Realization that intrauterine growth retardation 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.
Placental Health and Amniotic Fluid Levels: The placenta and amniotic fluid levels 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 impaired 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 Retardation: When this is observed as early as the second trimester, especially with associated oligohydramnios, it suggests that a potentially poor outcome to pregnancy is likely. (Fetal death, birth defects, mental retardation, and handicaps may occur in such instances.)
Polyhydramnios: Excess amniotic fluid volume, or 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.
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