An oddity I came across while researching something completely different.Q. Can a woman have two fetuses in different stages of development in the uterus at the same time?
A. It is possible for a second fertilization to occur when one pregnancy is already progressing. The condition is called superfetation, and while it is fairly often seen in mammals, including cats, only a few case studies have been reported in humans.
A French report of one apparent case, published in 2008 in The European Journal of Obstetrics & Gynecology and Reproductive Biology.said that a review of the scientific literature had found fewer than 10 such reports up to that time. “Superfetation is defined by the fertilization and the implantation of a second oocyte in a uterus already containing the product of a previous conception,” the report said.
In this case, each fetus had a separate amniotic sac. The difference in their size was obvious in the first trimester of pregnancy and continued, leading the doctor to offer superfetation as a diagnosis.
Ordinarily, the release of eggs ceases once a woman is pregnant, and the hormonal and physical changes of pregnancy work together to prevent another conception. If a second pregnancy does later occur, it often results in a risky premature birth. But in at least one American case, in Arkansas in 2009, both children were born healthy by cesarean section, though they were conceived more than two weeks apart.
https://www.nytimes.com/2017/02/27/science/superfetation-pregnancy.html
