I can't even imagine what's going through your head as you contemplate this important decision...especially as an expectant mother. First, health insurance is very expensive. Different employers offer different options and pay different amounts toward the premiums, but all in all it's expensive. I hope I can help with some understanding as it's also very complex.
Insurance usually comes in the form of an HMO (Health Maintenance Organization) or a PPO (Preferred Provider Organization).
HMO's typically are less expensive. They operate in that you select a primary care physician or medical group. For doctor visits you must see your primary provider or group. You can only see a specialist once you've been referred by your primary. Kaiser is an HMO. You visit their offices, see their doctors, get referred to their specialists, and have babies in their hospitals. They have had a bad reputation in years past, but I've heard wonderful things about them over the past decade.
PPO's give you the freedom to choose any doctor you desire and, in most cases, without a referral. Insurance will often pay more (80%-100%) toward your visits if you choose a "Preferred Provider", which means a doctor/group that has a contract with the insurance company and agrees to their terms. You can usually see doctors out of network, but will most like have to pay more out of pocket (50% or more).
Then, there are a lot of things to consider on top of that. Co-pays, which would be how much you have to pay each time you visit a doctor. Deductibles, which is how much you have to pay overall before insurance kicks in and starts paying for your visits. Emergency coverage, especially if you are traveling. Alternative care coverage (i.e. chiropractic, mental health, etc.).
Personally, my family has a PPO and I love it except for the expense of it. A couple of decades ago HMO's got a bad reputation...long story. At that time I switched to a PPO and built wonderful relations with many doctors, primaries and specialists. Now that HMO's have a better reputation, I'm not willing to give up my doctors. I love my OB/Gyn, my allergist, my pediatrician, and others that I have a history with. Plus, I love going straight to a specialists without seeing a primary first. For example, just last week I took my son to a podiatrist to treat a wart on his foot. My daughter had one once and the pediatrician could not attend to it because of its nature and referred me to a podiatrist. This time I went straight to the podiatrist and did not even bother with the pediatrician. As a result, it saves me money because I only have to pay 1 co-pay instead of 2.
Kaiser at one time had one of the worst reputations and they're still recovering from that. So, you'll sometimes hear negative feedback regarding them. But, overall I've heard wonderful things about Kaiser in recent years. If I had no history with anyone, I probably would choose that insurance option. But, if picking your own doctor to meet your needs is important to you , then I would investigate the PPO's.
A good source of information could also be provided by your company's Human Resources department. Someone there should be able to take the time to go through and explain your actual options so you can more easily understand it. Regardless, I hope that my information was helpful as I'm far from being an expert. I wish you luck in your decision.