I work in the insurance industry, though not in medical insurance. I sell Life, Long Term Disability, and Dental Insurance.
Dental and Medical plans both fall under health, and operate in the same manner.
If you are hearing the words Usual and Customary, you must be enrolled in a PPO (Preferred Provider Organization), and used a hospital that wasn't in the network. If you used a hospital in the network, you need only look at your EOB (Explanation of Benefits) and track down the "Member Responsibility (or some phraseology like that)" and pay that amount.
The way a PPO works is that your carrier contracts with a number of providers who agree to discount their rates in order to have more patients driven to them. In this arrangement, the carrier saves money by having you go in-network, as the cost per procedure is lower. The carrier will generally incent you to go in-network by offering higher levels of coinsurance, or lower deductibles in-network.
When you go out of the network, the provider you see is entitled to charge you whatever they want, and you are willing to pay. Your insurance company is indemnifying you, but you are selecting your provider and agreeing to pay their prices, as they're not contracted to abide by any provisions implemented by your insurance carrier. Because there is a possibility of a wild swing in pricing, insurance carriers use Usual and Customary as a way to benchmark reimbursements. (You'll also see this referred to as Reasonable and Customary).
U&C is calculated along percentiles, not percentages. Common U&C levels are 80th and 90th, though a wide swing is available and has a significant impact on your premium rates.
If you have a 90th percentile U&C out of network reimbursement, your claims will be paid at the lower of what 9 out of 10 providers in your area will charge for a service, or actual charges.
So, if you've got 10 doctors, and their charge for a given procedure is as follows:
$150
$99
$99
$98
$98
$97
$97
$94
$93
$85, you're going to be reimbursed based on a cost for that procedure being $99. If you go to the doctor who charges $150, you will be responsible for the extra charges. If you see any of the other doctors in the sample, your only charges will be your deductible or coinsurance that you're responsible for when going out of network.
So, looping back around. If you were in-network, go off of your EOB. Pay only the member responsibility total. If the hospital continues to harrass you for further payment, call BC/BS and inform them that the hospital is trying to balance bill you for in-network services that are not in accordance with what your EOB states.
If you were out of network, the balance above U&C falls squarely on your shoulders. As the judge would say, ignorance of the (contractual provision) is no excuse. The only way I could see you getting around this would be if you were far from home and the birth came at an unexpected time, and you had to get to the nearest hospital with no option to seek out an in-network hospital, or the hospital you planned to deliver the baby at.
If you planned the whole time to deliver the baby at the out-of-network hospital, and didn't look at your benefits, or call your provider during the 9 months you had to prepare for this, it's really nobody's fault but your own.
I hope that helps.