Twins

Fraternal twins result from independent conceptions of two ovum/sperm pairs, and it is like two separate pregnancies happening at the same time. Fraternal twin embryoes can be slightly different in size, because conceptions may be a few hours apart, and twin conceptions with some fertility drugs can be a few days to a week different in the occurrence of conception. Genetically identical twins result from a single sperm/ovum pair that subdivides after conception. When the split occurs right after conception, twins develop in separate sacs, after three days they share an apartment, and a very late split - more than a week after conception, portions of the bodies can be incompletely separated, resulting in "conjoined" twins , which are very rare, perhaps two cases per million pregnancies. Identical twins are EXACTLY alike. ANY size difference between twins presumed or known to be identical is a serious indication of a growth disturbance and must be evaluated and monitored closely.

Chromosome problems are slightly more common in twins than in single fetuses, and genetic amniocentesis is now recommended for twins, in the absence of any other risk factor, when maternal age is 33 years at the time of delivery. When there are two sacs (which is called a di-amniotic twin pregnancy) each sac is sampled. We usually schedule amnios for twins a little later than for singletons - 13 to 14 weeks GA, in preference to 11 to 13 weeks.

These are identical twins at 7 weeks GA (5 weeks after conception) in a single sac. One of the things we worry about in every multiple pregnancy is a problem called the twin to twin transfusion or the "parabiotic" syndrome. This happens when the umbilical circulations communicate with each other, enabling a siphon effect - one fetus gets too much blood flow, becomes fluid overloaded and eventually goes into heart failure while the other becomes dehydrated and malnourished. Parabiotic syndrome is most likely when there is one sac and one placenta and the cords have a common origin. In the picture above the cords are well separated, however bridging vessels may form between the cords leading to the same problem. Parabiotic syndrome can also occur in fraternal twins when bridging vessels are present, but this is very much less likely to happen than with identical twins, who have to share one placenta. We spend time looking for bridging vessels and assessing shunt flow in every case of twins, because it is possible to fix this problem in utero before circulatory problems become critical.

Choroid Plexus Cysts