Though heat may be a factor, another usual suspect definitely is not, according to Dan Caton, professor of physics and astronomy at Appalachian State University in West Virginia. He tested the old wives' tale (or old midwives' tale?) that babies come during full moons and found no correlation between the phases of the moon and births.
"If there were any true effect, it would have come out, and there's just nothing there," Caton said. "But the myth will outlive me."
Indeed, Roberts, the nurse manager for labor and delivery at Baptist Hospital in Miami, said her nurses still believe the legend: "When there's a full moon, when there's a storm brewing, we feel there are more people coming. Is it a proven fact? Of course not. … But if you ask most labor room nurses, they'll swear by it."
Caton thinks he knows why the perception defies the statistics.
"The moon you're most likely to see is the full moon because it rises at sunset and sets at sunrise," he said. If you're a maternity ward nurse, "you're going to remember those occasions, but you're not going to remember others -- the equally busy nights when you came out and you weren't distracted by the full moon. So the myth will live on."
Some suspect culture may contribute to the different birthrate cycles in different countries.
In religious Christian countries, for instance, the Christmas holidays might spur conception -- and, by extension, a fall birth surge like the one seen in the United States. However, at least one study noted fall birth surges in India and Israel, two generally non-Christian countries.
Perhaps Europe's cycle could be influenced by a greater amount of vacation and leisure time in the summer -- causing more summer conceptions and accounting for the spring surge in births.
Some have speculated people in certain countries may try to time their births for culturally auspicious times, to avoid inauspicious times. For instance, Japan saw a sudden dip in registered births in 1966, the year of the "elder fire horse" in that country's traditional 60-year cycle.
"There is a superstition that girls born in the year of the elder fire horse, 'Hinoe-Uma,' will kill their husbands," said Ron Rindfuss, a professor of sociology at the University of North Carolina-Chapel Hill, and a fellow at the Carolina Population Center. "Prospective parents, fearful about the marriageability of their daughters, prefer to avoid having a daughter born in the year of the elder fire horse."
There may even be evidence of a cultural event's short-term impact on births in America: Births of Southern white babies dipped for a time following Brown v. Board of Education, the landmark 1954 Supreme Court decision that racially integrated America's schools, according to a study by Rindfuss.
"That one, to me, at least makes good intuitive sense," Rindfuss said. "The world in which their [segregation-era Southern whites'] children would go to school potentially was radically altered."
Similarly, a Penn State University study found an increase in marriages, births and divorces in regions hardest hit by 1989's Hurricane Hugo, and Roberts believes her Miami maternity ward sees the effects of hurricanes nine months later.