Joan was presented as a woman who wanted to have lots of fun before she settled down and knew how to use her sexuality for her advantage (I mean gitfts Roger gave her, not that awful Jaguar guy that was suggested by Pete. But her fate also showed that women were in a different situation in having sex than men. As a single girl Joan had had two abortions (presumably before the pill in the 50ies) and when she was married, she got pregnant from Roger because they sudden life-affirming sex after being robbed (presumably she didn't use the pill then because she tried to get pregnant by her husband when he was on leave from the army). Her original decision was abortion but in the last minute she retreated and decided to use that old trick by presenting the child as her husband's. Which he accepted - but could still leave the child as easily as his wife. (Yeah, he met the baby only on one leave, but still - what a jerk. Compared to Greg, Don was a paragon of father.)
All in all Joan shows that great liberator of women, the pill, liberated men even more.
I think it was great that the show dealt the theme of unwanted pregnancy in so many ways. Peggy was single and chose adoption because she put her career first and didn't want to suffer of "one mistake" her whole life (and no doubt the child would have suffered, too, whether Peggy had said aloud that "you destroyed my life" or only thought so). A mother of two children, Betty, was married but separated from her husband and thought about abortion but took Don back. Megan got miscarriage before she had time to decide what to do - and before she even told to her husband. At that Don said that he "wanted the same as you", but later suggested to have a baby but Megan refused because her acting career.)
Compared these unwanted pregnancies, it was sad that just Trudy who so much wanted a child, had so much difficulties to get one. And Pete wasn't actually supporting and refused to adopt. Finally they got a baby and on the basis of what was shown to us, Trudy genuinely liked to be at home (unlike Joan and Betty) raising her daughter even if it was tiring in the beginning.
Oh, I almost forgot Stephanie who also got pregnat as singe and gave away the child she had no means and abilities to take care of.