Today I read an article about women on the same subject. From the Wall Street Journal:
"Women have some control over when they give birth, a study suggests—and they don't want to give birth on Halloween.
Researchers looked at all U.S. births, 1996-2006, during the two-week windows around Halloween and Valentine's Day—the theory being that connotations of hearts and candy were more pleasant than those of ghouls and goblins.
Indeed, "spontaneous" births rose 3.6% on Valentine's Day, compared with other days in the two-week window, while caesarian deliveries rose 12.1% and induced births 3.4%. But on Halloween, spontaneous births fell 5.3%, caesarians 16.9% and induced births 18.7%. The researchers said the rise and fall of spontaneous births suggested that women can "expedite or delay" them "within a limited time frame."
"Influence of Valentine's Day and Halloween on Birth Timing," Becca R. Levy, Pil H. Chung and Martin D. Slade, Social Science & Medicine (October)
So! Studies show that humans have a little bit of control over when they give birth. If women were having them in a pasture, I bet they'd show more of a preference for sunny days, too.