Their marriage has become the subject of intense speculation and scrutiny amid an embarrassing online sex scandal. Now, Representative Anthony D. Weiner and Huma Abedin are about to make news of a different kind: they are expecting their first child. Mr. Abedin, 35, is in the early stages of pregnancy, according to three people with knowledge of the situation. The pregnancy, which the couple has disclosed to close friends and family, adds a new dimension to questions about the future of their marriage.