Amnionic fluid fills the amnionic sac. In early embryonal life, during the first five weeks, amnionic fluid represents a transudate of maternal plasma. As the fetal circulation and the chorion develop, the amnionic liquor becomes a transudate of the fetal bloods (weeks 6–12). Later on, the surface ectoderm of the embryo differentiate into the epidermis, becoming impermeable, and the fetus swallows amnionic fluid and contributes urine. As the fetal kidneys ripen, the urine becomes more and more hypotonie. The swallowing of the liquor in the third trimester is considered to be ca. 500 ml daily (Abramovich, 1970). The volume of the amnionic fluid increase is estimated at 25 ml from week 11 to 15, and 50 ml from week 15 to 28. At the end of the fourth month, the volume is estimated at 150–250 ml; at 20 weeks, approximately 400 ml; at 24 weeks approximately 500 ml; at 30 weeks approximately 900 ml and at 35 weeks 1000 ml (graph 25). At the end of week 40 (the end of gestation), the average amount is about 650 ml. The osmolarity of the amnionic fluid decreases with increasing age and with increasing contribution by the fetal urine, dropping from 275–280 mOs at gestational weeks 4–12 to approximately 270 at week 20, 265 at week 30, 255 at week 35 and 250 at week 40. Osmotic damage of the amnionic epithelium is considered as one of the principal fetal factors timing the delivery (Jirásek, 1977).
KeywordsEstrogen Cortisol Testosterone Norepinephrine Progesterone
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