In the third trimester, what is the most common indication for amniocentesis?

In the third trimester, what is the most common indication for amniocentesis?

• A. Checking the fetal cells for chromosomal abnormalities
• B. Determination of fetal lung maturity
• C. Checking amniotic fluid for intrauterine infection
• D. Determination of whether alpha-fetoprotein (AFP) is present in the amniotic fluid

Answer: (B)

In the third trimester, amniocentesis is most often used to assess whether the fetal lungs are mature enough for breathing outside the womb. Testing the amniotic fluid for lung maturity centers on surfactant markers, especially the lecithin-sphingomyelin (L/S) ratio and the presence of phosphatidylglycerol. An L/S ratio around 2:1 or higher and the detection of phosphatidylglycerol indicate mature lungs, meaning the risk of respiratory distress syndrome after birth is lower if delivery occurs soon. This information is crucial when there’s concern about imminent delivery or when delaying delivery isn’t possible, guiding management decisions. Other options are less typical in the third trimester. Chromosomal abnormality screening is usually pursued earlier in pregnancy when such information is most actionable. Checking amniotic fluid for infection can occur, but it isn’t the common reason to perform amniocentesis late in gestation. Determining whether alpha-fetoprotein is present in amniotic fluid relates more to assessing neural tube defects or dating issues earlier, not the routine late-pregnancy use.

In the third trimester, amniocentesis is most often used to assess whether the fetal lungs are mature enough for breathing outside the womb. Testing the amniotic fluid for lung maturity centers on surfactant markers, especially the lecithin-sphingomyelin (L/S) ratio and the presence of phosphatidylglycerol. An L/S ratio around 2:1 or higher and the detection of phosphatidylglycerol indicate mature lungs, meaning the risk of respiratory distress syndrome after birth is lower if delivery occurs soon. This information is crucial when there’s concern about imminent delivery or when delaying delivery isn’t possible, guiding management decisions.

Other options are less typical in the third trimester. Chromosomal abnormality screening is usually pursued earlier in pregnancy when such information is most actionable. Checking amniotic fluid for infection can occur, but it isn’t the common reason to perform amniocentesis late in gestation. Determining whether alpha-fetoprotein is present in amniotic fluid relates more to assessing neural tube defects or dating issues earlier, not the routine late-pregnancy use.