| disease | Hypamnion |
| alias | Oligohydramnios |
A pregnancy in the advanced stage with an amniotic fluid volume of less than 300 ml is called oligohydramnios. Oligohydramnios in the early or intermediate stage (second stage) of pregnancy often ends in late abortion. When oligohydramnios occurs, the amniotic fluid becomes viscous, turbid, and dark green. In the past, the incidence of oligohydramnios was thought to be about 0.1%, but in recent years, due to the widespread use of B-mode ultrasound, the detection rate of oligohydramnios has increased to 0.5–4%, indicating a rise in incidence. Oligohydramnios significantly affects perinatal outcomes and has thus garnered attention.
bubble_chart Etiology
Since the mechanisms of amniotic fluid production and circulation have not been fully elucidated, the causes of many cases of hypamnion remain unknown. Clinically, the following conditions are commonly observed.
1. Fetal malformations: Such as congenital renal agenesis, renal hypoplasia, or stenosis of the ureter or urethra, which lead to oliguria or anuria, resulting in hypamnion.
2. Post-term pregnancy: During post-term pregnancy, placental function declines, perfusion is insufficient, and the fetus becomes dehydrated, leading to oligohydramnios. Some scholars also suggest that in post-term pregnancy, the fetus becomes overly mature, increasing the sensitivity of renal tubules to antidiuretic hormone, thereby reducing urine output and causing hypamnion. The incidence of hypamnion due to post-term pregnancy is 20–30%.
3. Intrauterine growth restriction (IUGR): Hypamnion is one of the characteristics of IUGR. Chronic hypoxia triggers a redistribution of fetal blood circulation, prioritizing the brain and heart while reducing renal blood flow, leading to decreased fetal urine production and hypamnion.
4. Amniotic membrane lesions: Electron microscopy reveals that the epithelial layer of the amniotic membrane thins in cases of hypamnion, with epithelial cell atrophy, shortened and thickened microvilli, swollen tips, reduced numbers, and squamous metaplasia. The rough endoplasmic reticulum and Golgi apparatus in the cells are also diminished, along with a reduction in desmosomes and hemidesmosomes between epithelial cells and the basement membrane. It is hypothesized that some cases of unexplained hypamnion may be related to intrinsic amniotic membrane lesions.bubble_chart Clinical Manifestations
Pregnant women often experience abdominal pain during fetal movement. Examinations reveal that abdominal circumference and uterine height are smaller than those of others at the same stage of pregnancy. The uterus is highly sensitive, and even mild stimulation can trigger contractions. After labor begins, the pain is intense, contractions are often uncoordinated, cervical dilation is slow, and labor is prolonged. If oligohydramnios occurs in the early stages of pregnancy, the fetal membranes may adhere to the fetus, leading to fetal deformities or even limb deficiencies. If it occurs in the middle or late stages of pregnancy, the pressure around the uterus directly affects the fetus, increasing the risk of musculoskeletal deformities such as torticollis, curved spine, or malformed hands and feet. Studies have confirmed that inhaling small amounts of amniotic fluid during pregnancy helps fetal lung expansion and development, whereas oligohydramnios can lead to pulmonary hypoplasia. Some scholars also suggest that for post-term pregnancies, intrauterine growth restriction, or pregnancies with hypertensive disorders, if fetal heart rate changes occur before active labor, the possibility of oligohydramnios should be considered. Oligohydramnios increases the risk of fetal distress and neonatal asphyxia, raising perinatal mortality rates. Statistics from Shanghai indicate that perinatal mortality rates are five times higher in cases of oligohydramnios compared to normal pregnancies. Therefore, it is one of the key conditions requiring prevention and treatment.
B-mode ultrasound diagnostic method In recent years, this method has made significant progress in the diagnosis of {|###|}hypamnion{|###|}, but there are still differing opinions on its diagnostic criteria. During {|###|}pregnancy{|###|} weeks 28 to 40, B-ultrasound measurements show that the maximum amniotic fluid pool diameter remains stable within the range of 5.1±2.1 cm. Therefore, a measurement method where the maximum amniotic fluid pool depth perpendicular to the {|###|}uterus{|###|} contour (AFD) ≤2 cm indicates {|###|}hypamnion{|###|}, and ≤1 cm indicates severe {|###|}hypamnion{|###|}. In recent years, the amniotic fluid index (AFI) method has been advocated. This method is more sensitive and accurate than AFD. An AFI ≤8.0 cm is used as the critical value for diagnosing {|###|}hypamnion{|###|}, while ≤5.0 cm is used as the absolute value for diagnosis. In addition to the amniotic fluid pool, B-ultrasound also reveals unclear interfaces between the amniotic fluid and the fetus, obvious contact between the placental fetal surface and the fetal body, and compressed and curled fetal limbs.
Direct measurement of amniotic fluid When the {|###|}membrane{|###|} ruptures, less than 300 ml of amniotic fluid is used as the diagnostic standard for {|###|}hypamnion{|###|}, characterized by its viscous, turbid, and dark green nature. Additionally, multiple round or oval nodules, 2–4 mm in diameter, pale gray-yellow, opaque, and containing stratified squamous epithelial cells and vernix caseosa, are often observed on the surface of the amniotic {|###|}membrane{|###|}. The biggest drawback of the direct measurement method is the inability to achieve early diagnosis.
The diagnosis can be made based on clinical manifestations and auxiliary examinations.
bubble_chart Treatment Measures
Hypamnion is a dangerous and extremely important signal for the fetus. If the pregnancy has reached full term, the membrane should be ruptured as soon as possible to induce labor. If the amniotic fluid is scanty and viscous, with severe meconium contamination, and fetal distress occurs simultaneously, and it is estimated that childbirth cannot be completed in a short time, cesarean section should be chosen to terminate the childbirth after excluding fetal malformations. Cesarean section can significantly reduce perinatal mortality compared to vaginal childbirth.
In recent years, the use of amniotic cavity infusion to prevent and treat advanced-stage hypamnion during pregnancy has achieved good results. One method involves placing a pressure-measuring catheter and a scalp electrode in the amniotic cavity during labor to monitor the fetus, and infusing 0.85% saline at 37°C into the amniotic cavity at a rate of 15–20 ml per minute until the fetal heart rate variability decelerations disappear or the AFI reaches 8 cm. Typically, relieving fetal heart rate variability decelerations requires infusing about 250 ml (100–700 ml) of saline. If the variability decelerations do not disappear after infusing 800 ml, the procedure is considered a failure. Amniotic cavity infusion can relieve umbilical cord compression, reduce the rates of fetal heart rate variability decelerations, meconium passage, and cesarean section, and improve neonatal survival rates. It is a safe, economical, and effective method, but repeated amniotic cavity infusions may lead to complications such as chorioamnionitis.
