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New Jersey cities have some of the highest C-section rates in the US. What about yours?

Fair Lawn, New Jersey, and Chestnut Ridge, New York, are, in most ways, quite similar suburbs. Both are within an hour's drive of downtown Manhattan. It takes a half-hour to drive from one to the other.

But there's at least one really big way Fair Lawn and Chestnut Ridge differ. In Fair Lawn, more than 70 percent of babies are delivered via caesarean section. But in Chestnut Ridge, the number stands at just 9 percent.

There's no data suggesting the patients in Fair Lawn and Chestnut Ridge are especially different. The census shows that both have similar demographics with predominately white, upper-income areas. Instead, the more likely explanation is that doctors practice medicine really differently in different cities — and those differences don't have much at all to do with the particular situation of their patients.

Health officials have, in recent years, become worried about the increasing prevalence of C-section births. While the procedure does make birth more predictable, it's also costlier and riskier than vaginal delivery. The risk of the mother dying in birth, while low in both procedures, is four times as high for C-sections as it is for vaginal births.

This is why the American College of Obstetricians and Gynecologists recommends vaginal births over C-sections for uncomplicated pregnancies.

The health data firm Amino health analyzed insurance claims for 4.4 million pregnancies over the past five years. What they found was huge geographic variation in how obstetricians deliver babies. It suggests that some doctors — like those practicing in Fair Lawn — aren't heeding the warnings on too-high C-section rates at all.

Some zip codes have C-section rates of 8 percent — and some have rates above 70 percent

Amino's data set can display particularly granular data on C-section rates, showing the number for every zip code in the country. Its map (an interactive version of which you can find here) shows huge variation in C-section rates across the country.

(Amino Health)

Most zip codes in America have a C-section rate between 28 and 40 percent, the dark blue and purple bars you see above. The average C-section rate among Amino's data set is 37 percent. (This is higher than the federal average of 26.2 percent, which Amino says might reflect a slightly different time period — its map covers 2010 to 2015 — and different methodology; for twins delivered via C-section, Amino would count that as two C-section births).

But there are also outliers. Five zip codes in the United States have C-section rates of 9 percent. Then there are 97 zip codes that have C-section rates of 70 percent or higher (Amino groups those outliers together).

Health care experts recommend against scheduled C-sections. Doctors do it anyway.

Scheduled C-sections can, practically speaking, make the business of having babies easier and more predictable. One co-worker recently told me about how her mother's doctor had scheduled her delivery around the Super Bowl, asking whether she'd like to have her baby before or after. Still, doctor groups generally recommend against this type of scheduling because of the increased risk associated with C-section procedures.

The most striking is this chart of births in December, which shows a significant decline in the C-section rate on Christmas. The big drop in C-sections around December 24 and 25 suggests that patients are scheduling procedures — and avoiding the days that they would, unsurprisingly, prefer to celebrate with friends and family.

(Amino Health)

The same type of trend showed up when Amino looked at C-section rates on weekdays versus weekends. Once again, there was a noticeable decline in C-section rates on the weekends.

(Amino Health)

"Scheduled C-sections tend to happen in business hours," says Jorge Caballero, Amino's medical director. "The unscheduled ones have the same cadence as [vaginal births]."

What should you do if you live in a high C-section area and are expecting?

What we know from the Amino data is that there's really big variation in C-section rates by city. What's impossible to know though, is how many of those are the right clinical decision — and how many go against professional guidelines.

There are lots of health disparities in the United States, with some pockets of the country where women might experience more complicated pregnancies than others. In these areas, we would expect higher C-section rates even if every doctor were following best practices. And the Amino data doesn't adjust for that; it just shows the raw numbers for C-sections versus vaginal births.

Caballero says he sees the especially high C-section rates as a good starting point for talking to one's own doctor and showing up to appointments informed. Patients can ask their doctors about their philosophy on C-sections — and shop around if they don't like the answer. "If my wife and I were expecting in one of those areas, I would use this information to have a conversation with our OB-GYN about why that may be the case," Caballero says. "This map doesn't give answers for why the high rates are happening, but it raises the specter that it needs to be addressed."

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