Hi DocDiva,
Here is advice on having babies from my perspective, take what you want, leave the rest.
It took me 6 months to get pregnant with one miscarriage. I was only three months pregnant in the "window" when I had "planned" the baby to be born. Trust me, these little people are your first lesson in plans not going according to plan. Since you are interested in OB, I'm thinking that is all the more reason to have one before residency since DURING is NOT a good time. If you definitely want a baby before residency, start trying NOW. It can take us young healthy women up to a year and still be considered "normal".
Once you have the baby, you have his warm little body cuddled up to you, breastfeeding, you might find that you don't want to leave him after a short 8-12 weeks. You need to be in position where you could have that option. Once you've signed the contract to start residency, its done... you start.
I love being a mom, more than ANYTHING. It transforms you and is completely worth the sacrifices that you will make gladly. Don't be surprised though if after having a baby, you find yourself reconsidering OB and looking at other lifestyle specialties.
I always thought I could pop out babies and let someone else take care of them but I was wrong. I love caring for my baby and I could not hand him off to a day care or nanny at 6 weeks. Its hard to leave him with a babysitter at 7 months. This is why I'm resolved not to have any more children until after residency.
I'm all for starting a family before residency, just know that you may love that baby more than ANYTHING in the the world. The most carefully laid plans.......