Caesarean Section and Fostering
There is great variation in the survival rate of microinjected eggs introduced into pseudopregnant recipient female mice. If the survival rate is low, then the few eggs that do develop tend to be “overnourished” and grow into larger fetuses compared to the embryos of a normal-sized litter. Under such circumstances, the overgrown embryos can sometimes fail to be delivered, because their size prevents normal passage down the birth canal. If pregnancy proceeds for 2 or 3 d beyond the normal gestation period of 19–21 d, there is a danger that the pups will die in utero. Such pups can be rescued by caesarean section and fostering. Fostering may also be needed to save the valuable transgenic, but unweaned, pups born normally to a mother that unexpectedly dies.