I have twin boys who are now 4 1/2. While it is tempting to just buy a convertible car seat from the beginning and forego the infant car seats, I wouldn't recommend doing that.
The infant carrier baby seats are essential when the babies are little. They will fall asleep in the seats and then you just click the seat out of the car and can bring them inside without waking them up. If you put them in a convertible car seat, you need to unstrap them, carry them in, get them into a crib or some other thing while you run out and get the other twin. It is just really difficult and you don't want to risk waking the babies up.
Second, the infant carrier seats can fit into a double snap n go, which I HIGHLY recommend for the few months. Yes, it is an extra expense, but boy, it was a life saver. If we took the boys anywhere in the car, we'd just click out the car seats, snap them onto the metal snap n go frame and be done. No lifting them out of the car seats to put them in a separate stroller. It's just so easy. And easy is what you want with twins.
Ultimately, around a year (or when they reach the max. weight requirement), you'll need the convertible car seat. I highly recommend the Britax Marathons (in case you are registering for those). We have them and they have lasted very well for the past 4 years. My daughter, who is 6, still has hers and barely any wear and tear....
Good luck with the rest of the pregnancy. Drink LOTS of water and stay off your feet as much as possible!