Transcript for Royal Baby: William and Kate Are Expecting
and once upon a time, all of england waited for the birth of a suitable heir to the thrown because in part in. Friend: The royal woman in question couldn't produce -- thanksfully times have changed since the tudors but hard to imagine any newlywed under as much pro creation pressure than kate middleton, after being stared at, we have the latest on the royal baby. ♪ Reporter: Ever since they walked down the aisle the source of endless fascination and speculation, when would william and kate produce a royal heir? Today, 20 months after the wedding, an answer. That's amazing. Wow, congratulations to them both. I'm quite overcome, it's raleigh exceca really exciting -- Reporter: It's emotional. You're happy for them? Oh, yes. Reporter: Last week, she was playing field hockey at her former school. This afternoon the news announced only after the duchess of cambridge was admitted to hospital with a condition called hyperemesis gravidarum, or hg. The palace today said as the pregnancy is in its very early stages her royal highness is expected to stay in hospital for several days and will require a period of rest thereafter. This is extreme, excessive, intractae vomiting or throwing up that is going on several times an hour all day long and potentially it is a very serious situation. Reporter: William spent several hours by kate's side, seen here leaving earlier this evening. And after centuries of rigid tradition, this royal child will be third in line for the throne, regardless of whether it's a boy or girl. Thanks to new legislation about to be passed by parliament. It is kate to provide an heir but she's not under pro sure to provide a boy anymore. This is the beginning of the super-modern monarchy, what the queen has devoted her life for. It is historically very important. Reporter: Tonight, there is even talk of twins, increasing the confusion over the line of succession. It was kate's diagnosis, hg can indicate a higher possibility of twins that started the speculation. We know that it tends to be more common in women carrying multiples so women pregnant with twins or triplets, you can see definitely some hyperemesis in those women. The third in line is the child born first, even if it's just by a couple minutes. Reporter: In any event, prince harry will be farther from the throne. still on military duty in afghanistan. From the moment they were enga engaged kate and william said they wanted children. I think one step at a time and get over the marriage thing first and look at the kids but obviously, we want a family. So, we'll have to start thinking about that. Reporter: Everyone else has been thinking about it as well. She kept the world guessing, looking for clues, water instead of wine, declining a peanut stroking a baby. The palace had to swing into action and release the information earlier than they probably had intended. Kate is less than 12 weeks pregnant. Prime minister david cameron got tipped off earlier today. I got a note in a meeting, and found it difficult to keep it to myself. Reporter: Three decades since the media and masses were in a frenzy about princess diana' pregnancy with william. Diana very charmingly told everyone and she said I've got the most dreadful morning sickness. Diana suffered from it too and totally, totally miserable at the beginning of her pregnancy. Reporter: This media will create a media storm, tonight already clamoring outside the hospital. And when it came to royal parenting, diana, they laughed together and tight bond that charles wasn't fortunate to have with his mother when he was a child, she was busy being queen, it was a different world and time. I think we'll see a lot more of the diana-style parenting from kate than the old-fashioned way charles had. Reporter: They spent much of the last year celebrating the queen's diamond jubilee as her ambassadors abroad in the 60th year of her reign. She was married a year, done what expected of her and wanted to strt a family as soon as possible. Reporter: There is so much speculating ahead. Boy or girl? What about a name? I want it to be named daisy after me. And finn. Minnie. Reporter: We have months to go. Let the wagering begin. For "nightline" I'm lama hasan in london.
This transcript has been automatically generated and may not be 100% accurate.