Traditionally, doctors use gestational age, the number of weeks from fertilization to birth, to determine whether a preemie should receive invasive care.
This picture taken on January 20, 2009 shows a newly born Madagascan lemur, Propithecus coronatus, an en-dangered species named Tahina, at Besancon Zoo, eastern France.
While some of this can be explained by society—in countries like China baby boys are favoured and many unborn girls are electively aborted—there are natural processes at work.
As they report in the Journal of Mammalogy, they found that as the female lechwe got older, the sex and the birth weight of their young steadily shifted.
Methods The sciatic and brachial plexus nerve of three-day-old SD mice was treated with collagenase type IV and trypsin to remove the desmocyte on the Schwann cell, then explanted in the culture disk.