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By Lisa Flam

Soon after Britain’s Prince George celebrated his first birthday came the news that another heir is on the way for the Duke and Duchess of Cambridge.

Do the math, and the royal couple will have their hands full soon with two children under the age of 2. Closely spaced siblings are a royal tradition: Princes William and Harry are just over two years apart in age. George and his little brother or sister will be even closer. Is it better to have siblings spaced closer together, or farther apart? Science says one thing — but what's right for each individual family will vary. 

Soon Prince George will have a non-furry sibling; he's pictured here with his parents and dog Lupo in an official portrait from last March. Handout / Today

American experts recommend women wait at least a year after giving birth to become pregnant again to improve the chances of a healthy baby. Close birth spacing — conceiving less than half a year after delivery — increases the risk of preterm birth, low birth weight and fetal death, says Dr. Allison Bryant, a maternal-fetal medicine specialist at Massachusetts General Hospital and an expert on birth spacing.

“Our best scientific evidence is that adverse outcomes are highest in women who wait less than 6 months between a birth and conception of the next pregnancy,” Bryant says.

Some things never change: He still loves to tease his older brother. Princes Harry and William with Princess Diana and Prince Charles in 1995; Harry and William are about two years apart. JOHNNY EGGITT / Today

The American College of Obstetricians and Gynecologists’ 2010 book “Your Pregnancy and Childbirth” calls for waiting a year to conceive again, while a study published earlier this year found that women who waited at least 18 months had the lowest rates of premature birth.

For the average woman who had a healthy pregnancy the first time, Bryant advises patients to wait until their child turns 1 before trying to conceive again.

“For patients who have more complicated outcomes, we have a different conversation,” she said.

When it comes to a family’s happiness, the best age spread between kids is determined more by the first child’s temperament, the family’s financial situation and feelings of readiness for another child than a set amount of time, said Eileen Kennedy-Moore, a family and child psychologist in Princeton, New Jersey.

“There’s no one right answer about the ideal spacing for children,” she said. “That’s a very personal decision that each family needs to figure out.”

Learning to share toys is part of the sibling experience, even for a prince, perhaps. Britain's Prince George holds a toy Bilby during a visit to Taronga Zoo in Sydney, Australia in April.POOL / Today

Psychologists commonly suggest a spread of four years, says Kennedy-Moore, when the older child is more independent and less likely to regress when a baby comes home.

“With a 4-year-old, it’s more like the mommy and older child sharing the baby rather than baby and older child sharing the mom,” Kennedy-Moore said.

While having children closer together can be exhausting for parents, it does get them through the baby stage quicker and the older child is less likely to feel jealous.

“He won’t remember what it’s like having Mom all to himself,” Kennedy-Moore said. “It will be like the baby was always there."

And siblings close in age may have stronger relationships.

“They may end up being closer friends because they have similar interests and have fun together, though there may be more rivalry,” Kennedy-Moore said. “More distance tends to mean less conflict but that less conflict could be due to a less close relationship.”

Gloria Miller with daughters Emmy and Olivia. The girls are 16 months apart in age.Today

For a child around age 2, a new baby’s homecoming is a massive change that takes getting used to.

“It’s very common for kids around that age to say, ‘Send the baby back,’” Kennedy-Moore said. “Even in the most loving families, they’re going to be asked to wait for a moment while I have to feed your sister or change a diaper. But that’s probably going to be less of an issue in the royal family.”

Gloria Miller of Boise, Idaho, loved growing up as one of five children who are all less than two years apart. She wanted the same closeness for her own family. Miller, 28, and her husband have daughters who were born healthy 16 months apart: Emmy, who turns 2 in November, and 6-month-old Olivia.

It can be stressful getting through each day, with double the diapers, but Miller loves watching the girls play and seeing the baby laugh at everything her big sister does.

“They’re the best of friends right now,” Miller said, adding: “Just watching them grow together is really awesome.”

Lisa A. Flam is a news and lifestyles reporter in New York. Follow her on Twitter.