Why do women put on weight during pregnancy? Mouse example

Photo: Sunflower, Joshrowe, Pixabay

Why do women gain more weight during pregnancy than the size of the fetus? Because of small intestine growth apart from the growing off-spring.

Although pregnancy is a natural stage for females and generally does not require specific treatment if both the mother and child are healthy, the bodies of many female animals, including humans, undergo significant changes during reproduction. For example, the heart shifts position upwards, while the urinary tract narrows and moves downward. Recent research from the Francis Crick Institute, published in the highest-impacted journal ,,Cell” , reveals that in mice, the small intestine may increase in length by approximately 18% by the end of pregnancy, a change similar to that seen in lactating females. Notably, this lengthening is already evident by day 7 of pregnancy in mice, according to the study’s findings.

Natural limits of reproduction

The entire pregnancy in mice lasts about 20 days, while women carry their babies for approximately 9 months. This difference is due to the longer development time required for human fetuses. Mice give birth to offspring that are not fully mature, which means that their young need more time and care to become independent, especially with their lifespan. In line with many studies, as scientists write in their paper, the intestine is a remarkably plastic organ: its epithelium is the fastest self-renewing tissue in mammals. In the small intestine, proliferating cells located within crypts give rise to cells that migrate upward into the villus and differentiate. It is a superficial paradox that slimmer women tend to get pregnant more easily than those with higher fat tissue. However, fat does impact fertility and thus the potential for conception.

The reproductive growth of the maternal intestine and other organs was suggested nearly 100 years ago and has since been shown to involve increases in intestinal length and absorptive area,

claim Irene Miguel-Aliaga and Tomotsune Ameku of The Francis Crick Institute.

What does it mean? Although the specific mechanisms underlying the process are not in-depth known, yet, reasons may include nutrients and microbiota. The latter ones are changing along with changes in the environment living organisms share and consume. And all of this brings consequences for reproduction action.

Is pregnancy evolving into a more energy-dense process?

Pregnancy is an effort for the female organism. Yet, previous studies have shown that remodeling of the maternal intestine is not confined to mammals. For example, as scientists write, the intestine of oviparous Drosophila fruit flies, which differs between males and females, grows and is metabolically remodeled in females during reproduction. What’s interesting, is that intestinal growth is anticipatory and genetically-controlled. In general, the gut tract’s work and shape are driven by specific hormones that play crucial roles in the digestive process as well as preserving the functions of the cells responding to a protein trigger obtained during millions of years of evolution. However, this does not imply some pieces remain completely stiff. Scientists observed accelerated migration of the enterocyte progeny up the villus in early pregnancy and, particularly, lactation. What’s new is that the researchers stated that the first pregnancy enables full reversibility of anatomical changes induced with pregnancy, while the subsequent ones make the process more difficult. Why? As scientists claim, villi grow during pregnancy by increasing the proliferation of their progenitors and the migration and size of differentiated epithelial cells.

Consistent with the dynamics of villus growth/shrinkage, these reproductive changes in epithelial cell dynamics are already apparent in early pregnancy, peak at the end of pregnancy, persist in lactation, and yet are fully and rapidly reversible post-lactation,

say Tomotsune Ameku and Irene Miguel-Aliaga of The Francis Crick Institute.

Experiments indicate that the remodeling of the villus is specific to reproductive status. According to scientists, the height, width, and depth of the villus—which is responsible for absorbing nutrients and eliminating unnecessary or harmful food components—return to their pre-pregnancy values. Notably, the height of the villus remains similar between the first and second pregnancies, in contrast to changes observed in gut elongation.