Examining the placenta may help identify the cause of most unexplained pregnancy losses, US researchers found, with signs of pathology in its tissues uncovered in more than 90 percent of their studied cases.

Of the approximately 5 million pregnancies in the US each year, around 20 percent fail prior to the 20 week milestone in what's known as a miscarriage, with a further 20,000 lost after this date in stillbirth.

It's not uncommon for patients whose pregnancies end in heartache to hear that their loss is unexplained, which can add to the pain and guilt they feel. About half of the pregnancy losses in the US have no known cause.

"The pressing clinical issue remains identifying the cause of the loss and employing methods of preventing future losses when possible," write Yale University reproductive scientist Beatrix Thompson and colleagues in their published paper.

A number of pregnancy losses are attributed to infections, or placenta issues that are more routinely diagnosed, like dysfunction of the umbilical cord or early separation from the wall of the uterus.

The placenta primarily connects the baby's own vascular system to the mother's via the umbilical cord during pregnancy, providing oxygen, nutrients, even lifesaving protective antibodies, while removing waste products.

Because it is so important, changes in this temporary organ can lead to miscarriage or stillbirth. The researchers investigated whether adding two categories of placental pathology to the existing recorded ones would help explain the losses.

"To have a pregnancy loss is a tragedy. To be told there is no explanation adds tremendous pain," says reproductive scientist Harvey Kliman from Yale University.

"Our goal was to expand the current classification systems to decrease the number of cases that remained unspecified."

Thompson, Kliman, and Yale University data scientist Parker Holzer examined a database of 1,256 pregnancies lost without explanation between 6 and 43 weeks of gestation. Miscarriage is defined as pregnancy loss before 20 weeks of gestation, while stillbirth is pregnancy loss at or after 20 weeks.

By examining placental pathology slides from each case, the team found that nearly 92 percent of cases could be diagnosed with a specific placental pathology. This included over 85 percent of miscarriages and almost 99 percent of stillbirths.

The most common pathologic feature seen in unexplained miscarriages was dysmorphic placentas, a marker associated with genetic abnormalities. In unexplained stillbirths the most frequent pathologic feature was a small placenta, which can restrict the baby's growth and development.

Due to the difficulty of safely evaluating this organ during pregnancy, doctors and researchers have historically overlooked the placenta. In 2009, Kliman was part of a duo that developed a safe method to measure the volume of the placenta. While the test takes just 30 seconds, most physicians still don't do it.

"This work suggests that the over 7,000 small placentas per year associated with stillbirths could have been detected in utero – flagging those pregnancies as high risk prior to the loss," says Kliman.

"Likewise, the identification of dysmorphic placentas may be one way to potentially identify genetic abnormalities in the almost 1 million miscarriages that occur in our country every year."

The large sample size and range of timing of losses give the research merit, though the study also has limitations. It was retrospective so it can't establish a causal relationship between placental pathology and pregnancy loss. And it was conducted at a single institution, so the results may not be generalizable to other populations.

But the findings could pave the way for more people to be able to understand the cause of their loss and make informed decisions about future pregnancies.

"Having a concrete explanation for a pregnancy loss helps [patients] understand that their loss was not their fault, allows them to start the healing process, and, when possible, prevent similar losses – especially stillbirths – from occurring in the future," Kliman says.

The research has been published in the journal Reproductive Sciences.