Arthur Miller, in his play The Crucible, "announces" Elizabeth Proctor's pregnancy late in the drama. During the Salem Witch Trials, pregnant women were not punished (hanged/pressed/put through the trials) until after they delivered their child. The pregnant women, instead, would stay in jail until their child was born.
One could assume that Miller "made" Elizabeth pregnant for a few reasons. First, he does not wish to have one of the loved characters in the play to die. Many readers feel sorry for Elizabeth (given her husband's affair) and see her to be a good person (based upon her characterization).
Second, the courts needed to try to use Elizabeth to force John to sign the papers. The court believed that Elizabeth could convince her husband to sign the papers (so he would not be put to death).
Lastly, the pregnancy shows promise for the relationship between Elizabeth and John. The fact that she is pregnant shows that she and John still have the type of relationship which could successfully bring a child into the world.