186 Obstetrics and Gynecology Board Review •••
❍ What is the mean amniotic fluid volume at term?
800 cc.
❍ What is the mean amniotic fluid volume at 42 weeks?
250 cc.
❍ What is the composition of amniotic fluid in the first trimester?
The electrolyte composition and osmolality are essentially the same as fetal and maternal blood.
❍ When does fetal urine first enter the amniotic sac?
8 to 11 weeks.
❍ Is amniotic fluid hypertonic, isotonic, or hypotonic in the third trimester as compared with
maternal plasma?
Hypotonic.
❍ What is the pH of amniotic fluid?
7 to 7.5.
❍ What is the pH of the upper vagina?
3.8 to 4.5—this difference in pH aids in determination of rupture of membranes.
❍ What causes amniotic fluid osmolality to decrease as pregnancy progresses?
Inflow of markedly hypotonic fetal urine.
❍ Under normal conditions, what is the average increase in amniotic fluid volume late in gestation?
30 to 40 mL/day.
❍ What are the six major pathways that help to regulate amniotic fluid volume late in pregnancy?
(1) Fetal urination.
(2) Fetal swallowing and reabsorption by the intestine.
(3) Secretion from the respiratory tract.
(4) Secretions from the fetal oral-nasal cavities.
(5) Intramembranous pathway.
(6) Transmembranous pathway.
❍ What are the three major determinants of amniotic fluid volume (when fetal anomalies are excluded)?
(1) Movements of water and solutes within and across the membranes.
(2) Physiologic regulation by the fetus of flow rates such as urine production and swallowing.
(3) Maternal effects on transplacental fluid movement.