Chelsea Clinton, from the moment of her difficult conception, was an enormous factor in keeping her parents together.

After her birth, far more complicated considerations were at work than the simple nostrum of parents who “stay together for the sake of the children”.

But Hillary’s and Bill’s devotion to their daughter was in its unconventional way (given the chaos and tension they generated) absolute and unconditional. Whatever the distractions, the family unit, when isolated from the outside world, functioned reasonably well. Bill and Hillary were both good at being hands-on parents.

Hillary had married for love, to have children and to create her idea of a model family. She felt she could figure out how to make the political part of her life work, but unless she could be a mother with a loving husband who was father to their children, life’s fulfillment would elude her.

She suffered, however, from a condition called endometriosis, which often makes conception difficult, can cause infertility and frequently results in extreme pain during and after intercourse.

From early in their marriage Hillary and Bill tried to have a child, and — famously — they even chose a name after hearing Judy Collins’s version of the song Chelsea Morning on a Christmas holiday in London in 1978.

Still unsuccessful after four years, they decided in the summer of 1979 to see a fertility specialist; but before the appointment Hillary found she was pregnant.

It was a worrying pregnancy and Chelsea was born three weeks early by caesarean section. Normal procedures forbade fathers in the delivery room when surgery was ordered, but Bill appealed to the hospital’s administrator, saying that Hillary needed him, and promised “that they could cut Hillary open from head to toe and I wouldn’t get sick or faint”.

They frequently referred to Chelsea as their “miracle child”. Hillary took four months’ maternity leave from her job as a lawyer and was sensitive to any criticism that she skimped as a mother. When a friend wrote and performed a song specially for Chelsea that included the words “We may not be worthy, but we’ll try to be wise”, Hillary seemed to bristle at the notion she was unworthy.

Friends of the Clintons were aware of her disappointment at not being able to have a second child. Even during their early years in the White House she and Bill talked seriously about adopting, and they discussed with friends in California who had adopted how they might go about the process themselves.

At the age of 48, Hillary raised the subject in an interview with a Time magazine reporter, saying: “I must say, we’re hoping to have another child.”

When the stunned journalist asked if she meant by natural birth, she added: “I have to tell you I would be surprised but not disappointed. My friends would be appalled, I’m sure.”

Having another child had been a recurring discussion in the Clinton marriage. “I think we’re talking about it more now,” Hillary said. “We’d obviously wait to get serious about it until after the [1996] election.”

This was before she’d heard the name Monica Lewinsky.

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