“Have you ever wondered,” one visiting professor asked, “why a colt doesn’t get up and gallop around inside the mare?” After all, a horse only minutes old is already able to hobble around the barnyard. The answer, as Mellor reported in an influential review published in 2005, is that biochemicals produced by the placenta and fetus have a sedating and even an anesthetizing effect on the fetus (both equine and human). This fetal cocktail includes adenosine, which suppresses brain activity; pregnanolone, which relieves pain; and prostaglandin D2, which induces sleep — “pretty potent stuff,” he says.
Combined with the warmth and buoyancy of the womb, this brew lulls the fetus into a near-continuous slumber, rendering it effectively unconscious no matter what the state of its anatomy. Even the starts and kicks felt by a pregnant woman, he says, are reflex movements that go on in a fetus’s sleep. While we don’t know if the intense stimulation of surgery would wake it up, Mellor notes that when faced with other potential threats, like an acute shortage of oxygen, the fetus does not rouse itself but rather shuts down more completely in an attempt to conserve energy and promote survival. This is markedly different from the reaction of an infant, who will thrash about in an effort to dislodge whatever is blocking its airway. “A fetus,” Mellor says, “is not a baby who just hasn’t been born yet.”