Dr. Harvey Karp is perhaps one of the most well-known names among pregnant women and new mothers. Karp is known for his book, “The Happiest Baby on the Block,” where he writes about the evolution of human babies and offers advice for new parents on how to soothe a screaming baby. One of the main theories in the book for why human babies scream so much and are so helpless is that they are born 3 months too early. But, because humans have a large brain-to-body ratio, babies must be born 3 months early—around 9 months instead of 12—in order to fit through their mother’s birth canal.
However, new research about the evolutionary history of human pregnancy might contradict Karp’s book, and offers further insight into why human pregnancy is so weird when compared to other animals. Dr. Vincent Lynch, associate professor of biology at the University at Buffalo and the study’s senior author, said in an interview that human pregnancy, labor, and delivery all last longer than it should. Additionally, Lynch said that the human placenta is invasive, burrowing further into the uterus than is seen in other animals.
The study, led by researchers from the University of Chicago, identified hundreds of genes that are turned either on or off during human pregnancy. Changes in the anatomy and physiology of the female reproductive system are key components in the origins of pregnancy in placental mammals, and these changes are ultimately controlled by a suite of genes.
Physiological functions controlled by some of these genes are constrained throughout pregnancy, and ultimately might contribute to adverse pregnancy outcomes. As such, the study sought to determine whether changes in gene expression during pregnancy are associated with any pregnancy pathologies.
Gene activity in the human uterus was compared to that of other animals and found that hundreds of genes were both gained and lost throughout human evolution. This includes genes that contribute to maternal-fetal communication (HTR2B), maternal-fetal immunotolerance (PDCD1LG2), and vascular remodeling and deep placental invasion (CORIN). These genes tend to affect adverse pregnancy outcomes like infertility, spontaneous abortions, pre-eclampsia, and preterm birth, and therefore the overall success and health of a pregnancy.
Moreover, results from the study found that over 900 genes are expressed during human pregnancy, and that some of these genes regulate immune and hormonal responses. One example was found in a serotonin receptor, and the authors argue that, perhaps, serotonin released from the brain during pregnancy could be involved in maternal-fetal communication. Further research may then explore the role of serotonin as it relates to labor, and the mechanisms through which it may influence the timing of birth. These results offer important insight for clinicians and evolutionary biologists as we all seek to better understand the weird and unique biology of humans.
Sources: The New York Times Magazine, UBNow, eLife