How does the body know it's time to give birth?
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Only about four percent of babies arrive exactly on their due date. So who presses the "it's time" button? Some say there's an alarm clock hidden in the placenta. Healthy, full-term babies can arrive up to about five weeks apart. Scientists have had three suspects: the baby, the placenta, and the mother's womb. Let's question them one by one.
The baby first. In the 1960s, a doctor in New Zealand ran experiments on sheep. He disabled a tiny signaling gland in the lamb's brain, and the ewe just stayed pregnant, never giving birth. Give the lamb a stress hormone instead, and the ewe gives birth early. So in sheep, the lamb itself calls the start. But the human placenta is missing a key step, so that trick doesn't work in people.
Mice gave an even neater clue: when the pups' lungs are nearly ready, they release a protein into the fluid around them. As if saying "I can breathe now." Do human babies do this too? Not proven yet. Next, the placenta. In 1995, a team in Australia followed 485 pregnant women, testing their blood for a hormone the placenta makes. It rises faster and faster, like a clock hand speeding up. Even more striking: one test at four or five months could roughly sort out who would deliver early and who late.
They called it the "placental clock." But that only shows it keeps time with the birth, not that it's the one pressing the button. Finally, the womb. All through pregnancy, one hormone keeps a foot on the brake, so the womb stays calm. In many animals, that hormone drops near the end. The brake lifts, and birth begins. People are different: on the day of birth, it's still high in the blood.
What changes is the womb itself: it slowly stops listening to the brake, like covering its ears. Its muscle cells grow more little channels so they can squeeze together, and grow more antennas for the "push" signal. So who presses the button? Most likely, no single one. The placenta's clock ticks, the baby reports "ready," the womb stops obeying the brake, and once the signals line up, labor starts. Once it starts, it can't stop: the baby's head presses down, and word reaches the mother's brain. The brain releases oxytocin, and the womb squeezes harder. The head presses harder, round after round, until the baby is out.
Worldwide, about one baby in ten is born too early. Figuring out these signals could help them stay in a few more weeks. Your guess: who calls the start first, the baby or the placenta?
Hardly anyone is born on the due date
Out of a hundred babies, only about four arrive exactly on their due date.
Healthy, full-term babies can arrive up to about five weeks apart. So who presses the "it's time" button? Scientists have had three suspects: the baby, the placenta, and the mother's womb itself.
Suspect one: the baby
In the 1960s, a doctor in New Zealand experimented on sheep. When he disabled a tiny signaling gland in the unborn lamb's brain, the ewe just stayed pregnant and never gave birth. When he gave the lamb a stress hormone instead, the ewe gave birth early.
In sheep, the lamb itself calls the start.
But the human placenta is missing a key step on that route, so the trick doesn't work in people. Mice gave an even neater clue: when the pups' lungs are nearly ready, they release a protein into the fluid around them, as if saying "I can breathe now." Do human babies do this too? Not proven yet.
Babies are busy in there long before birth: see why babies kick.
Suspect two: the clock in the placenta
In 1995, a team in Australia followed 485 pregnant women, testing their blood for a hormone the placenta makes. It rose faster and faster, like a clock hand that keeps speeding up and never stops.
One test at four or five months could roughly sort out who would deliver early and who late.
They called it the "placental clock." But that only shows it keeps time with the birth, not that it's the one pressing the button. The placenta sends the mother other messages too: see why pregnancy makes you sick.
Suspect three: the womb covers its ears
All through pregnancy, one hormone keeps a foot on the brake so the womb stays calm. In many animals, that hormone drops near the end: the brake lifts, and birth begins.
People are different: on the day of birth, it's still high in the blood.
What changes is the womb itself. It slowly stops listening to the brake, like covering its ears. Its muscle cells grow more little channels so they can squeeze together, and more antennas for the "push" signal.
Labor starts when the signals line up
So most likely, no single one decides.
The placenta's clock ticks, the baby reports "ready," the womb stops obeying the brake, and once the signals line up, labor starts.
Once it starts, it can't stop. The baby's head presses down and word reaches the mother's brain. The brain releases oxytocin, the womb squeezes harder, the head presses harder, round after round, until the baby is out.
Worldwide, about one baby in ten is born too early. Figuring out these signals could help them stay in a few more weeks.
Quick check
Near the end of a human pregnancy, what happens to the "brake" hormone that keeps the womb calm?
In many animals the brake lifts before birth; in people the brake stays on and the womb covers its ears.
