Current topic: water volume of the ovine conceptus; point of view.

The sheep is the only species for which there is sufficient information to justify an overview of maternofetal water transfer. Current information points to the following conclusions: (1) the site of water exchange between mother and conceptus is mainly the placenta; (2) the pressure that initiates transfer to the fetus is the osmotic pressure of solutes that are present in higher concentrations in fetal plasma than in maternal plasma, such as urea, fructose, amino acids, bicarbonate and lactate; (3) maternal ultrafiltrate thus attracted into the conceptus arrives there mostly denuded of electrolytes and is strongly hypotonic; thus it is the transfer of electrolyte that restrains the transfer of water; (4) maternofetal transfer of water is strongly facilitated by angiotensin I in the fetal circulation by a sequence of events that has not yet been elucidated; (5) the net combined osmotic and hydrostatic pressure that drives water across the placenta is of the order of only 20 to 80 mm Hg; (6) it is necessary to make a sharp distinction between transfer from mother to conceptus and transfer from one compartment to another compartment within the conceptus. Finally, the review lists certain precautions to be taken in the further study of water exchange between mother and conceptus.