Pregnancy lasts 40 weeks for a reason. At 35 weeks, a baby’s brain tips the scales at just two-thirds of what it will weigh by weeks 39 to 40. Going full-term gives a baby’s lungs time to mature and improves a baby’s ability to suck and swallow. But too many doctors — and moms — are disregarding advice from the American College of Obstetricians and Gynecologists to wait until at least 39 weeks to deliver.

Desire for convenience and an intolerance for the unpredictability of labor have resulted in thousands of babies being born too early, according to The Leapfrog Group, which tracks hospital quality.

It recently released data collected from 773 hospitals about the percentage of births between 37 and 39 weeks that were considered elective, or not medically necessary. The rates soared as high as 40% and varied widely between hospitals across the country and even in hospitals in the same city; in Los Angeles, for example, some hospitals reported rates as low as 4% and as high as 29%. (More on Time.com: Who’s Too Posh to Push? High Cesarean Section Rates Aren’t Moms’ Fault)

Now the March of Dimes is calling on hospitals to solve the problem by requiring every physician who schedules an elective delivery before 39 weeks to justify its medical necessity. “Doctors know it’s not right, but they just aren’t being held accountable,” says Alan Fleischman, medical director of the March of Dimes.

On one hand, it’s understandable. Doctors want to be able to better control their schedule, eliminating middle-of-the-night deliveries and ensuring that they — and not one of their partners — delivers a baby since the delivering physician often receives the bulk of reimbursement.

But while scheduling deliveries early may be convenient, it’s not the best choice for moms and babies. Inducing labor doesn’t always work and can wind up in a C-section delivery, which — though common — is still major surgery for the mother and may cause breathing difficulties for the baby. (More on Time.com: C-Sections on the Rise, Especially for Black Moms)

Babies who go through labor are born more alert and are better able to breathe and latch on at the breast. But C-section deliveries now account for one of every three births.

What qualifies as an early delivery? It’s somewhat open to interpretation because the definition of “full-term” is not absolute. It’s frequently considered 37 weeks, says Fleischman, but “true full-term is 39 or 40 weeks.”

To further complicate matters, gestational dating is largely inaccurate unless a woman has had a first-trimester ultrasound. Later ultrasounds can be off by two to three weeks, meaning an early elective delivery at 38 weeks might actually be taking place sooner than intended.

It’s kind of surprising that insurance providers haven’t curtailed the practice of early elective deliveries entirely as babies born sooner tend to have more health complications and cost more. Even babies delivered at 37 to 38 weeks can end up costing 10 times as much as a full-term newborn, according to the March of Dimes. One study found that reducing early elective delivies to under 2% could save close to $1 billion in health care each year. (More on Time.com: Closely Spaced Pregnancies May Contribute to Autism)

Leapfrog did single out some hospitals and health systems, such as Hospital Corporation of America, which have adopted policies to discourage doctors from scheduling C-sections and elective inductions for nonmedical reasons. Meanwhile, four other major health plans — Aetna, CIGNA, UnitedHealthcare and WellPoint — have joined together to spread the word that the final weeks of pregnancy should not be considered optional; they’re encouraging pregnant women to investigate the rates of elective deliveries at hospitals where they’re considering delivering.

Surveillance can work, according to a Wall Street Journal article about Utah’s Intermountain Health, which used electronic medical records to show doctors how early elective deliveries impacted health outcomes. (Babies born early for no medical reason spent more time in intensive care and relied more on ventilators than full-term babies.)

Using the carrot and stick approach, Intermountain Health established a protocol to deter early elective deliveries, then tracked those doctors who persisted anyway. There weren’t many: six months later, elective early deliveries dropped to less than 10% from 28%, and after six years it’s less than 3%.

“We have data from large programs and from single hospitals that you can change this practice,” says Fleischman, who is also a pediatric and neonatal specialist and professor at the Albert Einstein College of Medicine. “With peer review and accountability, we can change this.”

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