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Amniotic fluid index (AFI) is a quantitative estimate of amniotic fluid [1] and an indicator of fetal well-being. It is a separate measurement from the biophysical profile. [1] AFI is the score (expressed in centimetres) given to the amount of amniotic fluid seen on ultrasonography of a pregnant uterus.
Amniotic fluid is removed from the mother by an amniocentesis procedure, where a long needle is inserted through the abdomen into the amniotic sac, using ultrasound guidance such that the fetus is not harmed. Amniocentesis is a low risk procedure, with risk of pregnancy loss between 1 in 1,500 – 1 in 700 procedures.
Usual follow-up steps include (1) a prenatal ultrasound exam to look for fetal abnormalities and/or (2) measurement of AFP in amniotic fluid obtained via amniocentesis. Maternal serum AFP (MSAFP) varies by orders of magnitude during the course of a normal pregnancy. MSAFP increases rapidly until about 32 weeks gestation, then decreases gradually.
The presence of these biophysical variables implies absence of significant central nervous system hypoxemia/acidemia at the time of testing. By comparison, a compromised fetus typically exhibits loss of accelerations of the fetal heart rate (FHR), decreased body movement and breathing, hypotonia, and, less acutely, decreased amniotic fluid volume.
Some degree of weight gain is expected during pregnancy. The enlarging uterus, growing fetus, placenta, amniotic fluid, normal increase in body fat, and increase in water retention all contribute weight gain during pregnancy. The amount of weight gain can vary from 5 pounds (2.3 kg) to over 100 pounds (45 kg).
The lecithin–sphingomyelin ratio is a marker of fetal lung maturity. The outward flow of pulmonary secretions from the fetal lungs into the amniotic fluid maintains the level of lecithin and sphingomyelin equally until 32–33 weeks gestation, when the lecithin concentration begins to increase significantly while sphingomyelin remains nearly the same.
Positive fern test with amniotic fluid as seen under the microscope. The fern test is a medical laboratory test used in obstetrics and gynecology.The name refers to the detection of a characteristic "fern like" pattern of vaginal secretions when a specimen is allowed to dry on a glass slide and is viewed under a low-power microscope.
In the first trimester, the main sources of amniotic fluid are fetal lung secretions, transportation of maternal plasma across the fetal membranes, and the surface of the placenta. By the second trimester, the fetal kidneys start to produce urine which becomes the main source of the amniotic fluid for the remainder of the pregnancy. [4]