What's the problem with Yu-Ching Fei from Mars Intelligence? What does Yu-Ching Fei mean by his joke?

What is the content after Yu Qingfei's joke "Wife, I didn't see it tomorrow ..."?

My younger brother Yu-Ching Fei told the first joke about a gentleman who wanted to have sex with his wife in the evening, but his wife refused because he had a medical examination tomorrow. After turning off the lights, the husband who was insomnia and rejected sighed, and the wife couldn't bear it, so she turned on the lights. At this time, the husband said happily: Wife, I have never seen tomorrow's B-ultrasound ... oral cavity ... that's probably it!