Changes in bowel habits and odor during menstruation are common. Many people notice that their stool becomes looser, more frequent, and has a stronger, more unpleasant smell, often referred to as “period poop.” This shift in digestive function is a normal response to the hormonal fluctuations that govern the menstrual cycle. The temporary changes affect the smooth muscle of the digestive tract, altering how quickly waste moves through the system.
Hormones That Drive Digestive Changes
The digestive changes experienced during menstruation are primarily initiated by the fluctuation of progesterone and prostaglandins. Progesterone levels rise in the luteal phase, which has a relaxing effect on smooth muscles throughout the body, including the intestines. This higher level of progesterone slows down the movement of the gut, which can often lead to pre-menstrual constipation and bloating.
Just before and at the start of the period, the level of progesterone drops sharply. This sudden decrease removes the hormone’s relaxing influence on the bowel, contributing to the subsequent change in stool consistency. At the same time, the body releases prostaglandins from the uterine lining. These compounds trigger the uterine muscle contractions needed to shed the lining.
Prostaglandins are systemic and can travel through the bloodstream, affecting other smooth muscle tissues. The muscles of the intestines respond to these signals, leading to increased activity and contraction in the digestive tract. This dual action—the removal of progesterone’s slowing effect and the introduction of prostaglandins’ stimulating effect—is the root cause of the digestive changes at the start of the menstrual cycle.
How Increased Gut Motility Affects Stool
The increased activity in the digestive tract, or greater gut motility, means that food waste moves through the large intestine faster than usual. This acceleration is a direct consequence of the prostaglandin-driven intestinal muscle contractions. When the waste material is propelled more quickly through the colon, it spends less time in the large intestine.
A primary function of the colon is to absorb water from the forming stool before it is eliminated. With rapid transit time, there is insufficient opportunity for optimal water reabsorption, resulting in softer, looser stools or diarrhea. This change in speed and consistency is a hallmark of the “period poop” effect, manifesting as increased frequency and urgency of bowel movements.
The rapid transit can also temporarily affect the location and balance of the gut microbiota. The movement of this community can contribute to digestive discomfort. The less-formed nature of the stool, coupled with increased gut activity, sets the stage for the more pronounced odor that is often noticed during this time.
The Chemistry Behind the Strong Odor
The strong smell associated with period-related bowel movements is a matter of chemical compounds produced during digestion, amplified by the changes in gut motility. A significant contributor to foul-smelling stool is the presence of volatile sulfur compounds (VSCs). These compounds are naturally produced when gut bacteria break down proteins and undigested food components.
The faster transit time of waste through the intestine can lead to less complete digestion and absorption of nutrients. This leaves more residual protein and other organic material for the bacteria in the lower digestive tract to ferment. The rapid, increased fermentation process generates higher concentrations of these sulfur-containing gases, which are then expelled, intensifying the odor.
The hormonal fluctuations may lead to shifts in appetite, resulting in pre-menstrual cravings that can influence the bacterial activity in the gut. Increased consumption of foods rich in sulfur-containing proteins, such as red meat, or highly processed sugars can provide the odor-producing bacteria with more fuel. The temporary presence of menstrual blood in the lower bowel can also contribute a distinctive, metallic or coppery smell as certain gut bacteria metabolize the iron found in the blood.