Plenty of mammals give birth to a litter of offspring, but humans are typically limited to just one per pregnancy. That’s mostly because women typically only release a single egg cell when they ovulate. But what controls are in place to ensure one egg cell per cycle is the norm? To find out, researchers from Rice University used computer simulations to develop a new mathematical model, which they detailed in a paper published in Journal of The Royal Society Interface.
To give you a bit of a refresher on the hormonal dynamics of ovulation, follicle stimulating hormone (FSH) secreted from the pituitary gland usually prompts the selection of just one ovarian follicle (out of around 10 to 20) to release its egg cell into a fallopian tube.
Traditional views say that the largest follicle with the most FSH receptors gets selected. This new model suggests that the 10 to 20 follicles have an equal chance to release an egg. “Our model, which correlates well with real-world data, suggests that selection is completely random and still very precise,” study co-author Zhuoyan Lyu explained in a statement.
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According to the model, the first follicle is selected randomly when FSH levels reach a certain threshold. Once selected, the follicle begins producing estradiol, which inhibits FSH production in a negative feedback loop. When FSH levels fall below the threshold, no more follicles can be selected, however the timing is key.
“The model shows the first follicle selection event is most likely to happen when FSH rises only slightly above the threshold. FSH then drops quickly once a follicle is selected and estradiol is produced,” Lyu said. “In most cases, this short time interval limits the number of selected follicles to just one.”
In most cases, but not all. If the window when FSH levels are above the threshold is open long enough, a second follicle can also be randomly selected to release its egg, and that’s how fraternal twins are possible. This outcome is expected to happen less than 10 percent of the time, according to the model, and it is indeed quite rare—fraternal twins account for just 2 to 3 percent of births.
Researchers say this new mathematical vision of egg release can also help answer more questions about fertility. “For example, it could be that control of this FSH-estradiol cycle loosens slightly as women age, leading to increased probability of fraternal twins for women over 35,” co-author Anatoly Kolomeisky said. “Women with polycystic ovarian syndrome, who often have low FSH levels, may struggle with infertility because the FSH levels never reach the threshold to select a follicle for development.”
When math and science have a baby, the sky’s the limit. 
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Lead image: Rasi / Adobe Stock
