The Great Kate Wait: What It Means if the Royal Baby Comes After Its Due Date

Plenty of infants are born after the 40-week mark.

While the palace has not confirmed a specific due date, Middleton has said that she was due to give birth to the couple’s second child anytime between mid-April and the end of the month.

"Only 5 percent of babies are born on their due date," 40 weeks into a pregnancy, said Ashton. "Full term of pregnancy is 37 weeks to 42 weeks."

Dr. Kimberly Gecsi, an obstetrician and gynecologist at University Hospitals Case Medical Center in Cleveland, said if a woman is overdue, she should be seeing her obstetrician and medical team to ensure the fetus is developing and active.

"Between 39 and 41 weeks, really that’s the best time for both baby outcomes and mama outcomes," said Gecsi. "When you go past 41 weeks ... that’s when we start to see problems with not having enough fluid around the baby," among other issues.

Gecsi said as long as there are no other complications, a doctor can use medication to induce labor in an overdue pregnancy.

Ashton said the fact that the duchess already delivered a healthy baby indicates she likely will not have too much trouble during the second birth.

"The fact that she had a baby before is very, very reassuring," said Ashton. "We would expect that if she were to be induced ... she would successfully deliver."

ABC News' Katie Kindelan contributed to this report.