D.A.
I took a CAB, just hailed it and it was late at night - also I had to go home and then get another cab again the 2nd time in the middle of the night. It worked out perfectly. If cabs frequent your area, I'd just go w/ that.
And a car service will work fine as well - even if it takes 20 minutes to get there (besides the pain of the contractions), you'll be fine getting to the hospital within an hour's time from when you call them.