Trying to find proof of this story has proved extremely frustrating.
Supposedly, this story gets a Reuters article because of a new book about it by Jose Sandoval, titled "Mother Aged 5". But I can't find that book on amazon.com or barnsandnoble.com.
According to sexualrecords.com, this story was written up in the Dictionary of Medical Science in 1939. I have no idea what this is, but I am in a university library. Perhaps I could find it?
...
Argh! There are multiple "Medical Dictionaries", and they are all literally huge dictionaries of medical terms. I also found a set of encylopedias on reproduction, too. There proved to be so much information there, though, that I couldn't even begin to know where to look.
The whole "It was written up in the Dictionary of Medical Science" thing sounds like complete crap to me. Why would you write a story in a dictionary? Unless they meant it was a medical journal? But of course, there appears to be multiple "Journals of Medical Science".
The only thing I can say for certain is that Edmundo Escomel, who supposedly witnessed the events, is a real person. His name is all over the internet, mostly in medical websites written in Spanish. There's even a picture of him
here (halfway down the page).