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Fluid dynamics

The March of Dimes has this to say about oligohydramnios, the condition of having too little amniotic fluid (and thus too small a gestational sac):

The most important known cause of oligohydramnios early in pregnancy is birth defects in the baby (often involving the kidneys or other parts of the urinary tract) and ruptured membranes. The effect of oligohydramnios on the baby depends on the cause, the stage of pregnancy in which the problem occurs, and how little fluid there is.

In the first half of pregnancy, too little amniotic fluid is associated with birth defects of the lungs and limbs and increases the risk of miscarriage, preterm birth and stillbirth. [...] The causes of oligohydramnios are not completely understood. The majority of pregnant women who develop the condition have no identifiable risk factors.