170 
Monitoring Stem Cell Biseahoh 
embryos to divide still later than this* but cxscaslonally they do divide 
after the fourteenth day. These divisions may not be complete* and 
then the twins remain conjoined and can only be surgically 
separated after birth (Figure 80), The twin birth rate in the U.S. has 
increased markedly in recent yeais, and was 30.1 par 1,000 live 
births in 2001.® The rate of multiple births (most multiple births are 
twins; triplets and so on are more rare) is higher with assisted 
reproductive technologies and with higher maternal age. Dizygotic 
twins clearly can result in ART from transferring more than one 
embryo to the prospective mother. In addition, some assisted 
reproductive practices, like age of the embryo transferred, may be 
associated with more likelihood of monozygotic twinning,’ though in 
general the causes of monozygotic twinning are not known. 
Figure 8A-C: Modes of monozygotic twinning. [Carlson, 4* Ed., Fig. 1-12, page 23] 
VI, THE PRIMITIVE STREAK AND GASTRULATION 
PRE-PUBLICATION VERSION 
