Brace R A
Department of Reproductive Medicine, University of California, San Diego, La Jolla 92093-0802, USA.
Clin Obstet Gynecol. 1997 Jun;40(2):280-9. doi: 10.1097/00003081-199706000-00005.
Although there are fairly wide variations AFV normally undergoes characteristic changes across gestation in which it increases from 10-20 ml at 10 weeks gestation to average 800 ml at 24 weeks. Little change occurs from then until near term when AFV begins to decrease, and large decreases can occur in postterm pregnancies. Across gestation, 95% of AFVs are within the range of 1/2.57-2.57 times the gestational mean volume and 99% are within the range of 1/3.40-3.40 times the gestational mean. Although there are six pathways in which fluid and solutes can enter and/or leave the amniotic sac, there are only four primary pathways that contribute to AFV during late gestation. These include fetal urine and lung fluid secretion as the two primary sources of fluid, with fetal swallowing and intramembranous absorption as the two primary routes of amniotic water clearance. The intramembranous pathway also appears to be a primary source of amniotic solutes (e.g., sodium and chloride). Although fetal hypoxia has been widely believed to cause oligohydramnios, fetal hypoxic hypoxia and anemic hypoxia both appear to be associated with an increased AFV and polyhydramnios rather than oligohydramnios. It is speculated that the oligohydramnios associated with fetal hypoxia is caused by placental dysfunction in addition to the hypoxia.
尽管羊水体积（AFV）存在相当大的个体差异，但在整个孕期通常会经历特征性变化，即从妊娠10周时的10 - 20毫升增加到妊娠24周时平均800毫升。从那时起到足月前变化不大，足月前AFV开始下降，过期妊娠时可能会大幅下降。在整个孕期，95%的AFV在妊娠平均体积的1/2.57 - 2.57倍范围内，99%在妊娠平均体积的1/3.40 - 3.40倍范围内。尽管有六种途径使液体和溶质能够进入和/或离开羊膜囊，但在妊娠晚期只有四种主要途径对AFV有影响。这些途径包括胎儿尿液和肺液分泌作为液体的两个主要来源，胎儿吞咽和膜内吸收作为羊水清除的两个主要途径。膜内途径似乎也是羊水中溶质（如钠和氯）的主要来源。尽管人们普遍认为胎儿缺氧会导致羊水过少，但胎儿低氧性缺氧和贫血性缺氧似乎都与AFV增加和羊水过多有关，而不是羊水过少。据推测，与胎儿缺氧相关的羊水过少除了缺氧外，还由胎盘功能障碍引起。
