I've been researching "lorialets," moonlight-loving spritesdescribed by French fantasy author Pierre Dubois in his Great Encyclopedia of Fairies. Lorialets will have to be a post for another time; Dubois' Encyclopedia is not so much a collection of folklore as it is a guide to the world of his comics, and the only real-world sources he gave for lorialets were the Chroniques Gargantuines or Grandes Chroniques Gargantuines. These are a group of 16th-century chapbooks, not to be confused with the famous Gargantua books by Rabelais. I haven't been able to track these down yet. However, my research along the way took me into some fascinating superstitions about mooncalves.
The belief was that the moon influenced congenital defects. "Mooncalf" was a word for a "monstrous birth."
The historian Preserved Smith suggested that the mooncalf was a translation from the German Mondkalb. In December 1522, a deformed calf was born in Saxony. People thought that the folded skin on its head looked like a monk's cowl, and within a month, a popular new broadside compared the creature to controversial contemporary Martin Luther. The calf's birth was supposed to be a divine sign pointing out the unnatural Luther. It was dubbed the "monk-calf," which Smith suggested was a pun on Mondkalb. An English version referred to it as a "Moonkish Calfe," pretty good evidence for a pun. And this was big news, in part because Martin Luther quickly fired back with a pamphlet saying that the monk-calf symbolized the evil of the Church. Not long after this, the word "mooncalf" started to become popular.
Farther back in history, Pliny's Natural History spoke of "molas" - hard, lifeless masses of flesh, which it was believed a woman conceived on her own without a man. This is where you get the term "molar pregnancy." In Thomas Cooper's Thesaurus Linguæ Romanæ & Britannicæ (1565), Pliny's "mola" was interpreted as "moone calfe." Not too much later, in 1601, Philemon Holland translated Pliny's work as The History of the World, commonly called the Natural Historie of C. Plinius Secundus. Holland also chose to render "mola" as "moonecalfe."
To modern eyes, there's not much connection from Pliny's "mola" to the British authors calling it a "moonecalfe." But if the "Moonkish Calf" was fresh in the author's memory, it makes more sense.
Alternately (and perhaps not exclusively), J. W. Ballantine suggested that "calf" did not mean a baby cow, but a swelling, like the calf of the leg. (Calf coming from a word meaning "to swell" is an established theory.) "Moon" would come from the associations with menstruation found in Pliny. So mooncalf could mean, in Ballantine's theory, "menstrual lump." A 1676 German work used the word "monkalb" or "mutterkalb."
In the early 17th century, mooncalf became a popular term for either a monster, or a fool - this second similar to lunatic, from "luna". Shakespeare used the word for the monstrous character Caliban, from The Tempest, written around 1610; the misshapen Caliban was born to a witch who could control the moon. Chapman's Bussy d'Amboise (1607) calls women "the most perfect images of the Moone (Or still-unweand sweet Mooncalves with white faces)."
There were, in fact, superstitions about what the moon might do to pregnant women. In Breton superstition, if a woman or girl urinates outside under the moonlight, she runs the risk of giving birth to a monstrous being. An account is given of such a thing happening; upon being born the monster scurried beneath the bed, and people killed him with a stick. A second anecdote mentions a Breton servant woman who declared that she had never been with a man, and didn't know how she could have fallen pregnant unless it was the moon's influence. (Revue des Traditions Populaires, xv. (1900) p. 471.)
To sum up: there is a long history of superstition that the moon could influence pregnancy, either causing women to conceive monsters on their own, or creating congenital defects.
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Researching folktales and fairies, with a focus on common tale types.