Michelle Duggar and her 19th child, Josie, who was born 15 weeks early.

Michelle and Jim Bob Duggar  were hoping for child number 20 but the laws of nature were against them, according to medical experts. After publicly announcing her pregnancy, the 45-year-old had a miscarriage.

At a routine doctor's appointment, the couple - stars of  "19 Kids and Counting" - learned that she had lost the baby in the second trimester. The family is now resting at home and asked for privacy.

Of the nearly 6 million pregnancies each year in the United States, approximately 15 percent end in miscarriage, according to the Centers for Disease Control and Prevention.

But Duggar's advanced age, rather than the fact that she had 19 children, probably put her at greater risk.

As women age, chromosomal abnormalities are more common, according to Dr. James Woods, chair of the department of obstetrics and gynecology at the University of Rochester in New York. "It's not necessarily that she had one too many pregnancies."

"That doesn't mean a 45-year-old is incapable of carrying a pregnancy," said Woods. "More and more, we look at multi-factoral components in how the egg and sperm unite into that magic transformation to become a person."

In about half the cases, a cause cannot be determined. Among the conditions usually linked to miscarriage are advanced maternal age, chromosomal abnormalities, structural problems, infections, autoimmune disorders or a condition that causes the blood to clot in the placenta, known as thrombophilia.

"As one gets into post 35, 40, 45 age range, you have a greater and greater risk in the sperm to egg fertilization, to the blastocyst [differentiation], to the uterus," he said. "There is a greater risk of genetic abnormality - some lethal to the pregnancy itself … That's why we do genetic screening."

Michelle Duggar has had a previous miscarriage in her second pregnancy, according to People magazine. And their last child, Josie, was born 15 weeks early by Caesarian because her mother had preeclampsia, the number one cause of maternal death around the world.

Duggar said that she and her husband will name the child before a burial service.

"I feel like my heart broke, telling my children," she told People. "We have all been so excited about this baby and looking forward to April coming around and having a new little one in our arms. That was the most difficult. The Lord is the giver of life and he can choose when that life is ready to go on and be with Him."

Women grieve deeply, no matter how many children they have had,  according to Woods, who is author of "Loss During Pregnancy and the Newborn Period."

"The psycho-social issues are easily brushed aside among their audience because they are transfixed on the fact that she had 19 kids," he said. "They think miscarriage means nothing.  That's where medicine has failed women in the past."