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Chapter: Obstetrics and Gynecology: Maternal-Fetal Physiology

Maternal Physiology: Gastrointestinal System

The anatomic and functional changes in the gastrointesti-nal system that occur during pregnancy are due to the combined effect of the enlarging uterus and the hormonal action of pregnancy.

Gastrointestinal System


The anatomic and functional changes in the gastrointesti-nal system that occur during pregnancy are due to the combined effect of the enlarging uterus and the hormonal action of pregnancy. These changes produce a number of pregnancy-related symptoms that can range from mild discomfort to severe disability.



The primary anatomic change related to pregnancy is the displacement of the stomach and intestines due to the enlarging uterus. Although the stomach and intestines change in position, they do not change in size. The liver and biliary tract also does not change in size, but the portal vein enlarges due to increased blood flow.





Functional changes in the gastrointestinal system are the result of the hormonal action of progesterone and estrogen. Generalized smooth muscle relaxation mediated by progesterone produces lower esophageal sphincter tone, decreased gastro-intestinal motility, and impaired gallbladder contractility. As a result, transit time in the stomach and small bowel increases significantly—15% to 30% in the second and third trimesters, and more during labor. Additionally, the imbalance between the lower intraesophageal pressures and increased intragastric pressures, combined with the lower esophageal sphincter tone, leads to gastroesophageal reflux. Reduced gallbladder contractility, in combination with estrogen-mediated inhibition of intraductal transportation of bile acids, leads to an increased prevalence of gallstones and cholestasis of bile salts. Estrogen also stimulates hepatic biosynthesis of proteins such as fibrinogen, ceruloplasmin, and the binding proteins for corticosteroids, sex steroids, thyroid hormones, and vitamin D.




Some of the earliest and most obvious symptoms of preg-nancy are noted in the gastrointestinal system. Although energy requirements vary from person to person, most women increase their caloric intake by about 200 kcal/day. Nausea and vomiting of pregnancy (NVP), or “morn-ing sickness,” typically begins between 4 and 8 weeks ofgestation, and abates by the middle of the second trimester, usually by 14 to 16 weeks. The cause of this nausea is unknown, although it appears to be related to elevated lev-els of progesterone, human chorionic gonadotropin, and relaxation of the smooth muscle of the stomach. Severe NVP, which is known as hyperemesis gravidarum, can result in weight loss, ketonemia, or electrolyte imbalance.


Many patients report dietary cravings during preg-nancy. Some may be the result of the patient’s perception that a particular food may help with nausea. Pica is an espe-cially intense craving for substances such as ice, starch, or clay. Other patients develop dietary or olfactory aversions during pregnancy. Ptyalism is perceived by the patient to be the excessive production of saliva, but probably represents the inability of a nauseated woman to swallow the normal amounts of saliva that are produced.


Symptoms of gastroesophageal reflux typically become more pronounced as pregnancy advances and intra-abdominal pressure increases. Constipation is common in pregnancy and is associated with mechanical obstruction of the colon by the enlarging bowel, reduced motility as else-where in the gastrointestinal tract, and increased water absorption during pregnancy. Generalized pruritus may result from intrahepatic cholestasis and increased serum bile acid concentrations.



The two most notable gastrointestinal pregnancy-related physical findings are gingival disease and hemorrhoids.Although the incidence of dental caries does not change with pregnancy, the gums become more edematous and soft during pregnancy, and bleed easily with vigorous brushing. On occasion, violaceous pedunculated lesions, called epulis gravidarum, appear at the gum line. These lesions, which are actually pyogenic granulomas, some-times bleed very easily, but usually regress within 2 months of delivery. Rarely, excessive bleeding may occur, requiring excision of the granuloma. Hemorrhoids are common in pregnancy and are caused by both constipation and elevated venous pressures resulting from increased pelvic blood flow and the effects of the enlarging uterus.




Some markers of hepatic function may be altered during pregnancy. Total serum alkaline phosphatase concentra-tion is doubled, mainly due to increased placental produc-tion. Serum cholesterol levels increase during pregnancy. Although total albumin increases, serum levels of albumin are lower during pregnancy, primarily due to hemodilation. Levels of aspartate transaminase, alanine transaminase, γ-glutamyl transferase, and bilirubin are largely unchangedor slightly lower. Serum amylase and lipase concentrations are also unchanged.


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