Maternal plasma hypertonicity is accentuated in the postterm rat.
OBJECTIVE
In humans and rats, pregnancy-associated maternal plasma volume expansion and plasma hypotonicity may facilitate maternal-to-fetal water transfer. Although reduced amniotic fluid volume occurs commonly in postterm pregnancy, the mechanisms are unknown. We previously demonstrated a reversal of pregnancy-induced maternal plasma hypotonicity that occurs in the near term (20 days) pregnant rats. We sought to determine whether the relative maternal plasma hypertonicity continues in the postterm period.


METHODS
Rat gestation (normal, 21 days) was prolonged with subcutaneous progesterone injection. Pregnant rats at gestation, 18 days, 21 days, and 24 days and nonpregnant rats were studied. Maternal and fetal hematocrit levels, plasma osmolality, electrolyte levels, and amniotic fluid volume were determined. In addition, maternal and fetal tissues were analyzed for water and electrolyte content.


RESULTS
Compared with term (21days), postterm pregnant rats (24 days) had a significant increase in maternal and fetal plasma osmolality (293.7+/-1.4 mOsm/kg vs 302.8+/-3.7 mOsm/kg and 301.0+/-2.0 mOsm/kg vs 310.3+/-3.2 mOsm/kg, respectively; P<.01) and sodium and chloride concentrations. Conversely, both maternal and fetal hematocrit levels decreased significantly in the postterm period. Postterm rats demonstrated an increased fetal mortality rate (24%) and a significantly reduced amniotic fluid volume (4.2+/-0.6 mL vs 6.6+/-0.6 mL, P<.01).


CONCLUSIONS
These results indicate that the near-term reversal of maternal plasma hypotonicity that has been observed previously is further accentuated in the postterm pregnancy. This continued hypertonicity may induce a fetal-to-maternal water flow and contribute to postterm oligohydramnios and increased fetal morbidity and mortality rates.
Ladella
Subhashini J
SJ
Perinatal Research Laboratories, the Department of Obstetrics and Gynecology, David Geffen School of Medicine at University of California, Los Angeles and Harbor-UCLA Medical Center, 1000 W. Carson Street, Torrance, CA 90509, USA. mikeross@ucla.edu
Desai
Mina
M
Cho
Yong
Y
Ross
Michael G
MG
D000653:Amniotic Fluid / Q000378:metabolism; D000818:Animals; D001158:Arteries; D001769:Blood / Q000187:drug effects / Q000378:metabolism; D001921:Brain / Q000378:metabolism; D002712:Chlorides / Q000097:blood; D005314:Embryonic and Fetal Development; D005260:Female; D005312:Fetal Blood / Q000737:chemistry; D005313:Fetal Death / Q000453:epidemiology; D006400:Hematocrit; D006982:Hypertonic Solutions; D009994:Osmolar Concentration; D009997:Osmotic Pressure; D010920:Placenta / Q000378:metabolism; D049590:Postpartum Period / Q000097:blood*; D011247:Pregnancy; D011270:Pregnancy, Animal / Q000097:blood*; D011374:Progesterone / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000097:blood
D002712:Chlorides; D006982:Hypertonic Solutions; D011374:Progesterone; D012964:Sodium