Why is a fetus almost always upside down?
As a kid, I would try to hang upside down on the monkey bars but within minutes, I would feel my face become warm, red and light headed from the pressure like feeling as blood goes towards my head. I can’t hang upside down for more than a few minutes, so how does a fetus remain in an upside down position for months?!
A fetus does not actually take the upside down until at 20 weeks earliest, and it is protected by amniotic fluid. The amniotic fluid is liquid made up of water, hormones and antibodies that acts as a cushion (shock absorber) for the fetus from bumps, injuries and even gravity. It also prevents compression of the umbilical cord which is a crucial factor to the fetus’ positioning, supplying blood, oxygen, nutrients and acting as a waste removal system.
Additionally, over 95% of all fetuses turn upside down before delivery. But how is it that a fetus “knows” to turn upside down for a normal birth? Fetal programming and genomic imprinting may be linked to fetal positioning by resource provision and restriction in early development. Fetal programming is the theory that environmental cues play an important role in development and genomic imprinting is an epigenetic phenomenon that analyzes how your behaviors and environment can cause changes that affect the way your genes work.
While studying gene regulation in human pregnancy is difficult, there has been research on mice gestation that shows a set of genes essential for placentation, the formation of placenta. As the placenta provides the intrauterine nutrients needed for fetal growth, there are genetic cues that have a common location for placental development. Epigenetic regulation of the placenta evolves over the period of pre-implantation development and the pregnancy period. Epigenetic marks like DNA methylation and histone modification affect gene expression and result in genomic imprinting for proper fetal and placental development.
Fetal programming genetic factors cause the placenta to attach to the wall of the uterus and the fetal umbilical cord arises from the center of the placenta and latches onto the interior side of the mother’s uterus. A well developed placenta, and umbilical cord that if effectively able to provide nutrients and blood to the fetus leads to a larger baby with optimal birth weight and body length and body composition. THis is because the placenta sends signals to modulate fetal development through selective environmental pressures, which is where genomic imprinting and epigenetic landscape, for example of the GAB1 gene for signaling growth factors and cytokines, comes into effect.
This is fairly complicated but in summary, there is a combination of environmental and genetic factors during gestation to allow for fetal safety and development up until a natural normal birth. This includes the amniotic fluid that protects the fetus against gravitational forces, adverse effects from external forces such as gravity, provide a nourishing environment despite the rise in fetal urine towards late pregnancy stages, and reduces umbilical cord pressure. It simultaneously assists in supporting the fetus in the upside down position, which is the ideal position for a natural birth and naturally develops this way due to imprinted genes and fetal programming.
https://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1008709
https://pubmed.ncbi.nlm.nih.gov/20959349/
https://www.frontiersin.org/articles/10.3389/fendo.2019.00940/full
https://my.clevelandclinic.org/health/articles/9677-fetal-positions-for-birth
