Re: During a pregnancy the baby urinates and defecate into the birth sac ...

Thanks for your great question!  I particularly had fun researching this 
topic and learned some new things in the process.

The human embryo develops in the mother's uterus within a fluid-filled sac 
called the amnion.  The primary function of the amnion is to create a 
roomy, weightless, protective chamber in which the fetus can grow freely.  
Thus, as the embryo grows, the amnion expands as well.  The amount of fluid 
contained in the amnion (known as amniotic fluid) is important for 
maintaining the size of the cavity.  If there is an inadequate supply of 
amniotic fluid (a condition given the fancy term, oligohydramnios), and 
consequently an unusually small amniotic sac, the growth of the fetus will 
be constrained.  This can result in severe malformations.  The lungs, for 
instance, seem to be very sensitive to the amount of space available in the 
amnion.  If there is insufficient amniotic fluid, the wall of the mother's 
uterus will compress the chest of the fetus, restricting the development of 
the lungs.  Aside from cushioning the fetus and providing a medium in which 
he/she can move easily, amniotic fluid serves other important functions.  
For instance, it may be a source of fetal nutrients and, as we have already 
learned, is vital for fetal lung development.  

So, where does the amniotic fluid come from?  A portion of this fluid is 
supplied by the mother.  However, beginning at about 16 weeks of pregnancy, 
a substantial fraction of the amniotic fluid comes from the kidneys of the 
fetus in the form of urine.   If the fetus fails to excrete urine into the 
amniotic sac (this occurs when both kidneys fail to develop and no urine is 
produced), this results in a severe shortage of amniotic fluid 
(oligohydramnios).  In addition to contributing urine to the amniotic 
fluid, the fetus also consumes this fluid by swallowing it.  This liquid 
enters the fetal intestines where most of it is absorbed into the fetal 
blood.  Here, it can be processed into urine or, returned to the maternal 
blood via the placenta.  Therefore, the reason that the amniotic sac does 
not become overly distended with fluid is because the volume is kept in 
balance by opposing activities.  On the input side, there is fetal urine, 
as well as the liquid contributed by the mother which make up the volume of 
amniotic fluid; on the output end, the fetus swallows much of the amniotic 
fluid, a portion of which is returned to the mother, and some of which is 
excreted into the sac as urine.  As we have already discussed, this 
delicate balance can be disrupted when the fetus does not produce urine, 
resulting in too little amniotic fluid.  However, the converse also occurs: 
there can be an excess of amniotic fluid (a condition referred to as 
polyhydramnios).  This may result when the fetus fails to swallow amniotic 
fluid, sometimes due to malformations of the throat and esophagus.      

The fetus actually produces very little feces (more correctly referred to 
as 'meconium') while in the womb.  The small amounts that are produced are 
excreted into the amnion by the fetus only late in gestation.  A large 
amount of meconium in the amniotic fluid is frequently a sign of 'fetal 
distress'.  It is especially dangerous to infants during delivery because 
the thick meconium can block the airway and prevent the infant from 
breathing in oxygen if it is not removed.  In addition, once the baby is 
out of the uterus and takes its first breath, it may inhale small amounts 
of meconium into its lungs.  This is a particularly serious situation as it 
can result in serious illness in the newborn.    

I hope you find this information helpful.  Please feel free to contact me 
if you have any further questions!

Nikki