Sarah Jessica Parker told her son he could only have sex once

She admitted: "At that time I told him that he had one seed.

"And there was a young woman named Mary who he would meet in sophomore year of college. She was going to be very smart, and lovely, and come from a nice family. And that he would choose Mary to give his seed to, but he had only one."

However, it turned out the lie wasn't necessary as the 50-year-old actress - who, as well as James, now 12, also has twins Marion and Tabitha, five, with her spouse - had misheard what the youngster had asked.

She explained to talk show host David Letterman: "He had a little cloth that he called his 'baby' and he would hold it up to his face, and he asked me, 'Where does my baby come from?'

"I said, 'Oh my god! Where do babies come from?! I'm not prepared!' I thought I had a good 10 or 12 years!

"So I thought about what I would like for him. Which is that he has one seed, and that he should choose wisely."