Marc, I used the 50mm f1.4 on the D300, and I stopped the lens a bit, because I learned my lesson from the birth of my first son where many of my pictures came out blurry because of the narrow depth of field.Thanks to everyone that has responded so far.
bdelva: What body did you use? Did the baby come during the day or night?
Then it's safe to assume shooting at 1.8 with either the 35 or 50 would be the way to go. I'm not that afraid of pushing the ISO on the D300 that much. Hopefully it has slightly better ISO performance than the D40.
My other concern shooting at 1.8 is the narrow DOF. I dont want half the baby's face blurry. I know I could just step back. Problem is I'd rather stay as close to the wife for support of course. Dont want her thinking I'm just there as a photographer .
Ok now....what are the advantages (if any) between the 35 or 50.
Back then I had the D70s for only a couple of months, so I barely new what I was doing...as far as pushing the ISO I doubt that you will go above ISO400, and I did not start taking any pictures until the baby was laying under the heating lamp.
That gave me a chance to stay next to my wife for support, and depending on the setup of the hospital you might not eve stay in the same room after the delivery.
In my case they showed the baby briefly to my wife, and then we were off to another room where they check the baby and did the weighing.
D300 w/50mm f/1.4 @ 3.2 1/160sec ISO400
Hope this helps, and I'm sure you'll do fine...don't forget to post some pictures.