In which abstinence is effective 99% of the time, but to be completely safe, you should never eat anything. Or breathe.
I'm gaining a new appreciation for fairytale collections that include notes on the stories. Without it, it can be near-impossible to track down more information on the fairytales. This is what I went through with the Vietnamese Thumbelina, Nang Ut ("Miss Little Finger") for a while.
When I found a Vietnamese version, there was one thing that really surprised me . Namely, Nang Ut is a mother. A prince visits her garden, eats one of her melons, and throws the rind away. After she eats from the rind as well, she becomes pregnant. The prince eventually returns to her and decides to stay with her and their son. This kicks off the Doll-in-the-Grass plot of bride tests.
Nang Ut's birth is unusual, and so is the birth of her son. However, she is a "petit monstre," while her son is a miraculous child. Nang Ut's parents pray for her birth and undergo an unusually long pregnancy which results in an incredibly tiny daughter. This is in contrast to the German Thumbling, who is born after an unusually short pregnancy. Thumbling's story suggests premature birth, but Nang Ut is more reminiscent of a stone baby, who dies but remains inside the mother's body. While Thumbling's parents are loving, Nang Ut's parents abandon her because of her small size and monstrous nature.
Her son's size is unclear, or not remarked on. She's able to care for him, and in Landes' version she can hold him in her arms. In a visual adaptation on YouTube, however, the boy is of average size. Unlike Nang Ut's parents, the prince accepts her. Their child unites them instantly.
Food and drink as the cause of supernatural pregnancy is an incredibly widespread motif. A woman is told to eat some flower or herb, and the resulting child is often extraordinary in some way. (We see this again and again in thumbling tales. Three-Inch's mother eats a cucumber; Bitaram's mother steals a fruit; Pinoncito's mother is given a piñon nut.) Edwin Sidney Hartland has a huge list of variants, jumping off from the tale of Perseus, where Zeus takes the form of a golden rain to visit Danae.
A more specific variant, which might lend a little more internal logic to "eating from the same melon causes pregnancy," is that the food has been contaminated by a man's... um... bodily fluids, whether that's tears, saliva or something else.
In the earliest printed version I could find (a French translation by A. Landes in 1886), a footnote explains the implication that the prince made water into the watermelon's rind before throwing it away. Hartland mentions a similar tale, "The Lazy Man," also from Vietnam.
Some modern versions, like the folk opera on YouTube, cut out the pregnancy entirely and simply have the two meet in the watermelon garden. In a clever variation, Vuong has the rind accidentally strike Nang Ut when it's tossed away, so that the two meet immediately.
On another note, Nang Ut is an example of the Animal Bride tale, type 402 - very similar to Doll i' the Grass and Terra-Camina. I've commented on this before, but these characters are all associated with nature. Doll-i'-the-Grass with, um, grass, and Terra-Camina with earth. Nang Ut continues the pattern. She's a gardener. Her identity is tied to this place where she lives, surrounded by vegetation, so that her story shows up under titles like "Little Finger of the Watermelon Patch" or "Thumbelina in the Bamboo Tube" (Nàng Út Ống Tre). Although in these variations the bride is not an enchanted animal, she is still a being connected to nature.
This website is based on my research into folklore.