How did Lu Xiaoman know it was Wang Geng when she was pregnant?

Because Lu Xiaoman and Wang Geng had only had sex once before, and it was two months ago, it can be confirmed that the father of the child is Wang Geng. In the TV series "Good Time on the Move", Lu Xiaoman went to the hospital for examination after pregnancy, and the doctor told her that she was pregnant for two months, that is, early. There was only one relationship before, so she was sure that the father of the child was Wang Geng. The play tells the story of China from the end of1970s to the beginning of1990s, focusing on the experience of ordinary families and showing all aspects of China society in this period, such as family changes, cultural changes, social atmosphere and so on.