Had a look at all suggestions.
I don't think that it's metritis, while it could be a variant spelling of Melas, this was not identified in the 1800s, Mefruside is a fairly modern drug. Some other possible variant spellings of diseases were not identified until 1920's (such as Melorheostosis could have been written as "Melrius" - Dripping Candle Wax disease).
After looking at all the sections of the site below, there doesn't seem to be any obvious possibilities apart from the fact that "mel" is latin for sweet / honey.
http://www.antiquusmorbus.com/english/EnglishD.htmThe above describes Diabetes as "An immoderate or morbid flow of urine. It is termed insipidus ("tasteless") where the urine retains its usual taste, and mellitus ("honeyed") where the saccharine state is the characteristic symptom. [Thomas1875]
A disease which is attended with a persistent, excessive discharge of urine. Most frequently the urine is not only increased in quantity, but contains saccharine matter, in which case the disease is generally fatal. [Webster]
Diabetes is first recorded in English, in the form diabetes, in a medical text written around 1425."
I would agree with Kathb person who thought it might be diabetes (mellitus version) Mel = sweet / honey and is derived from the word mellus. I assume from the print on the certificate that the deceased died in Wales, and "Melys" is Welsh for sweet, diabetes being "y clefyd melys" the sweet disease. Diabetes can be brought on during pregnancy. As others have said, I would think that the the place of death would have a bearing on the accent / dialect of Welsh being spoken that would have caused the informant to be misunderstood, as well as a general lack of understanding perhaps of what the doctor had diagnosed (especially if it were a Latin term).
There is also Milroys disease (though it seems it wasn't named until 1892, but was identified before that in 1863 see
http://en.wikipedia.org/wiki/Milroy's_disease). This could be a possibility.
Personally I would go with mellitus - which the informant may have understood to have been "melrius" (which is what I think is written)I'm not sure why there would have been a large gap between the death and its registration, if this was the case.
Have you tried looking up the deceased's details in a local paper to see if there's anything there that might help.
Sam