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Labor Day

"This would be a good day to have a baby," my husband, George, said cheerfully one August morning, late in my pregnancy. He had it all planned out. It was the first day of school for our three older children (first grade, kindergarten, and a toddler program). With the kids away, he reasoned, he could shepherd me through labor with undivided attention and then later round them up to meet their new brother or sister for dinner.

Just one catch: It was still three weeks until my due date. The only labor I saw ahead was my job and the laundry.

The saga of birth begins deep in a fetus's brain, according to the latest research. So maybe this baby will turn out to be a smart (or obedient) child. Because around 2 o'clock that afternoon, the painless Braxton-Hicks contractions I'd been feeling for weeks began to knock the wind out of me. By 2:30, George was arranging for friends to pick up our kids. By 3, he ushered me into my doctor's office. And at 4:01 -- after I'd barely struggled into my hospital gown -- our daughter Page was born, in time to meet her siblings for supper.

What prompts a baby to suddenly tunnel out of the womb she's been snuggling in for months? Can you do anything to help labor begin, besides point out a convenient window on your calendar? Anyone who's ever counted down a due date can't help but wonder.

In the Beginning

My friend Rosemary swears by her neighbor's meat loaf. During her two pregnancies, her friend delivered the dish, and within 24 hours Rosemary was in labor. Another pal tried spicy pizza, long walks, and awkward late-term sex, all for naught. "I've had patients tell me they got on a trampoline or walked for three hours in the mall," says Glade Curtis, M.D., an ob-gyn in Salt Lake City and coauthor of the best-selling Your Pregnancy Week by Week. "Patients have a lot of interesting ideas about how to make labor start. Unfortunately, most of them don't work."

That's because labor is like a runaway train: You're along for the ride, but you have little control over when you leave the station or when you arrive at destination Motherhood. For nine months, it may seem as if your baby's a passenger, but it turns out that he's the one wearing the conductor's cap.

"For centuries, no one knew if it was the baby saying 'I am mature enough to survive in the outside world' or the mother saying 'I've had enough of you now: Out you go,'" says Peter Nathanielsz, M.D., an obstetrician and scientist who directs the Laboratory for Pregnancy and Newborn Research at Cornell University. "We now know that the baby actually initiates the events that result in a normal birth."

For nearly 40 weeks, a baby bides his time, growing and waiting for his limbs, lungs, kidneys, and nervous and blood systems to mature properly. He can't live outside the mother until then. "When all the systems are ready," Dr. Nathanielsz says, "the fetus sends a signal to Mom: Let's get on with it."

It's something of a miracle that the mother tolerates her baby-to-be as long as she does. After all, a fetus is technically a foreign invader, Dr. Nathanielsz says. It has a completely different genetic makeup. It takes oxygen and nutrients from its mother's blood, makes her first nauseated and later uncomfortable, and stretches her uterus to many times its normal size. But instead of trying to get rid of this rascal (as it would attack germs, for example), the mother's body indulges the baby, thanks in large part to progesterone. The hormone relaxes the uterus during pregnancy, preventing it from contracting hard enough to expel the fetus. It also keeps the cervix tightly shut.

So instead of asking how to make labor begin, you might ask how to remove the factors that have maintained the pregnancy. In the 1960s, research on lambs that refused to be born provided part of the answer. Their mothers had eaten a toxic type of corn lily, which caused their fetus's brain to be deformed. The ewes were abnormally overdue -- the equivalent of a human pregnancy lasting 15 months. Scientists eventually determined that the fetal lambs' diseased brains were incapable of sending the necessary signals to begin the birth process.

In the early 1990s, Dr. Nathanielsz and his colleagues pinpointed the origin of that signal in the brain's hypothalamus gland. Finally, the total domino effect of birth could be mapped out, at least in sheep, and the same is widely believed to be true in humans. Several days before the mother realizes she's in labor, the fetus's hypothalamus decides that all systems are go. It sends a message to the pituitary gland, also in the brain, to release stress hormones into the fetus's blood. The stress hormones make their way to the placenta, which orchestrates a cascade of hormonal changes in the mother: More estrogen! More oxytocin! These hormones, which have taken a backseat to progesterone throughout pregnancy, finally take the lead.

Because of all these changes, Mom's uterus contracts freely. Her cervix softens and dilates. Her baby -- who set off this chain of events while she was still oblivious -- is born.

Once Things Are Under Way

So if labor is tripped by a flurry of cell-level conversations, where does that leave a mother's actions? There's still much we don't understand.

Take the folk wisdom about having sex to trigger contractions. Some women have reportedly tried this tactic upon the advice of family or friends. And lo and behold, a few hours later, labor actually did kick in.

Just coincidence? Not necessarily, says Dr. Curtis. "Because they release oxytocin, both arousal and orgasm have been shown to make women have contractions. So can the chemical properties in the prostaglandins in semen."

What sex can't do is tell your fetus's brain to tip over that first domino -- that is, to ripen your cervix. That's why intercourse is safe during a normal pregnancy. However, precisely because it can increase contractions once a uterus is receptive, sex is discouraged in pregnancies in which preterm labor is a risk.

Some women report that nipple stimulation alone can lead to powerful contractions. When labor stalled during my second child's delivery, my ob nurse recommended it, and things did pick up. Rubbing the nipples releases oxytocin, which stimulates contractions. "There's some concern, however, about doing it just to start labor when you're not being monitored in a hospital," says Dr. Curtis. Since labor can proceed quickly once it has started, he doesn't recommend the practice.

Walking is another oft-quoted spark for labor, but there's no evidence that it can cause contractions. "Most of the time, labor is going to happen when it happens, and if you do something, it's just coincidence," says Ellen Troisi, a childbirth educator in Plainview, NY. But walking has other pluses. It's ideal exercise, even in late pregnancy. Once you're in labor, Troisi says, moving around has been shown to speed progress, perhaps because it helps the mother stay upright and relaxed.

Your Due Date: Just a Guess

There's nothing magical about the 280th day of pregnancy. "Only one in twenty women delivers on her due date," Dr. Curtis says. "Ninety percent deliver the week before or the week after." So D day is a two-week window -- not a 24-hour one.

Although a delivery anytime between 37 and 42 weeks is considered full-term, by the 40th week, doctors tend to monitor the pregnancy more closely to make sure the baby is still doing well. It's not known why some full-term babies don't initiate birth. Even if the baby is healthy, the human placenta begins to fail around 41 weeks, Dr. Nathanielsz says, which is why an ob may suggest labor induction when a baby's overdue. With inadequate nourishment and oxygen, the fetus is in danger.

What about babies who are born too soon? Most prematurity is due to infection or other factors -- such as smoking -- that set off inflammatory chemicals that produce prostaglandins and trigger contractions, says Stephen Fortunato, M.D., director of maternal fetal medicine at the Women's Hospital at Centennial Medical Center, in Nashville, TN. Other causes include structural problems with the cervix or placenta or high blood pressure in the mother.

Born just three weeks early, my daughter Page was considered full-term and normal. It's not likely that she read my thoughts, but since birth is still shrouded in many mysteries, who knows? However it starts, as Dr. Nathanielsz says, "birth is one of the most beautiful bits of biology there is."

Contributing editor Paula Spencer is the author of the Parenting Guide to Pregnancy and Childbirth.

The Start of Something Big

I was eating dinner at a nice Italian restaurant. The waiter was very slow, and my husband joked, "I bet I could get him to move a little faster by saying, 'Did your water break?'" Ten minutes later, as I was walking down the stairs out of the restaurant, my water actually did break.

Marisa Cohen, New York City

I was about a week overdue with my first child when my doctor told me that having an orgasm would be a natural way to stimulate birth. That night my husband and I made love, and I went into labor almost immediately!

Shelbi Baldridge, Taylor, MI

I was pacing on the deck waiting for some friends, who had promised to watch my older child, to arrive. They had stopped for bagels -- of all things -- and by the time they came, my contractions were ten minutes apart. On the way to the hospital, my husband and I unfortunately got stuck behind a dairy truck on a hill. There was an enormous picture of a cow's rear end on the back that I focused on while I did my Lamaze breathing!

Marjan Perry, Valley Cottage, NY