Gastrointestinal Tract
The gastrointestinal (GI) tract of the fetus grows rapidly in the first trimester, and by 12 to 13 weeks of pregnancy, some abdominal organs have completed three 90-degree rotations inside (and outside) of the body before finding their permanent locations in the abdomen.
The esophagus is formed early, and fetal swallowing of amniotic fluid has been observed as early as 16 weeks of pregnancy, which helps the rest of the GI tract continue to grow and develop.
A woman's proper diet, routine prenatal appointments, and glucose screening at mid-pregnancy can help limit the insulin produced by the fetal pancreas to prevent negative effects of fetal high blood sugar.
Additionally, and importantly, knowledge of the development of the fetal liver illustrates why any amount of alcohol during pregnancy can potentially harm the fetus – even if harm from a "minimal amount" (not defined) has not yet been consistently detected.
Background
The development of the gastrointestinal tract (GI) in humans has been documented for more than 100 years.
The GI tract begins to develop during the 6th week of pregnancy and largely completes all structural-related components by 13 weeks. Functional capability continues to develop throughout pregnancy as well as after birth.
Stomach and Intestines
The origins of the stomach, intestine, liver, and pancreas start developing during weeks 5 and 6 of pregnancy.
The yolk sac detaches into the body cavity of the embryo and becomes the primitive gut/gut tube. The portion of the yolk sac that's not included in the gut tube will become part of the umbilical cord.
There are three distinct regions of the gut: the foregut, midgut, and hindgut, which extend the length of the embryo and contribute to different parts of the GI tract.
The foregut will include the mouth, esophagus, trachea, respiratory tract, stomach, liver, gallbladder, and pancreas.
The midgut will include the lower duodenum, leading to the first two-thirds of the transverse colon, jejunum, ileum, cecum, appendix, ascending colon, and first two-thirds of the transverse colon, which all develop between 6 and 10 weeks.
The hindgut will include the last third of the transverse colon, descending colon, sigmoid colon, and rectum.
During the 7th week of pregnancy, the back of the stomach grows faster than the front, which is what creates the familiar curve shape of the organ.
As the midgut grows during the 8th week – which includes massive growth of the liver – there is not enough room in the abdominal cavity to accommodate it, so part of the intestines get pushed into the umbilical cord.
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By 12 to 13 weeks of pregnancy, the abdomen has enlarged so that the entire length of the midgut can fit, so the intestines return to the abdominal cavity. The small intestine returns first, followed by the large intestine. The exact mechanism that brings the intestines out of the umbilical cord and back into the abdomen is not completely known.
In approximately 1 in 10,000 births, the intestines do not return inside the abdomen (known as congenital omphalocele) and remain inside the umbilical cord.
Sometimes the intestines return correctly, but when they herniate again, they go through an umbilicus (belly button area) that was not completely closed. This causes a herniation, but it does not come out of the skin/tissue. About 95% of umbilical hernias correct themselves by the time a child is 5 years old.
When a baby is born with its intestines completely on the outside of the body, this is known as gastroschisis, and can be caused by a defect in the abdominal wall.
The small intestine is largely completed by the end of the first trimester. The intestinal surface area will undergo an approximate 100,000-fold increase from the early first trimester to birth.
Between 8 and 12 weeks of pregnancy, the midgut undergoes a total of three 90-degree rotations counterclockwise before organs are in their final location.
Esophagus and Swallowing
The esophagus begins to form during the 6th week of pregnancy with the formation of the foregut and can be observed by the 7th week.
The esophagus is initially short and must grow in length with the overall growth in length of the embryo; it rapidly grows longer as the heart and lungs relocate and reaches its final length by 9 weeks of pregnancy.
The esophagus and trachea initially start as one tube, and must split into their respective organs between 6 and 10 weeks; the molecular process of how this occurs is still not known, but ultimately results in an infant's ability to eat and breathe. A disruption of this separation can result in birth defects such as esophageal atresia or tracheoesphageal fistula.
The fetus swallows amniotic fluid as early as 16 weeks, which helps in the formation of the GI tract. Disruption in fetal swallowing is associated with excess amniotic fluid, known as polyhydramnios.
It has been estimated that during the final stages of pregnancy, the fetus swallows up to 400 milliliters (ml) of amniotic fluid per day.
Total stomach volume ranges from 30 ml in newborns to 2 liters in adults.
Reflux has also been shown to occur in utero and is a common symptom of preterm infants. Contents of the stomach of the fetus have been observed via ultrasound traveling all the way up into the pharynx.
Liver, Gallbladder, Pancreas, and Spleen
The liver, gallbladder, pancreas, and biliary duct system all start in the 6th week of pregnancy. The spleen follows at 7 weeks.
The pancreas starts as two separate buds that eventually fuse together to create one pancreas at the end of the 8th week.
Although insulin secretion begins at 12 weeks, the pancreas is relatively immature at birth, and undergoes substantial development after birth.
The liver grows rapidly from 7 to 12 weeks and fills a large part of the upper abdominal cavity. This lack of room in the abdomen is considered a factor in umbilical hernias (described above).
Despite the liver being one of the last organs to develop, by 11 weeks of pregnancy, it accounts for approximately 10% of the total weight of the fetus.
Alcohol can cross the placenta and enter the amniotic fluid, which the fetus can swallow and/or “breathe” in. The fetal liver has oxidative enzymes by the second month, but they are not active. Therefore, if alcohol is swallowed by the fetus, it would remain in the fetus for an extended period (read Alcohol).
Action
Women should avoid drinking during pregnancy. Although research has not yet identified harmful effects from a "minimal amount" (which has not been defined), based on knowledge of the development of the liver, GI tract, respiratory tract, amniotic fluid consumption, and how alcohol can be absorbed by the fetus, any amount of alcohol could therefore cause adverse effects. This includes subtle neurological effects that are very difficult to identify.
Women should read more regarding Alcohol consumption during pregnancy.
Women who have any questions or concerns (or something they do not understand after an ultrasound) regarding the development of their babies' GI tract should talk to their HCP.
Resources
Gut Development (Duke Medicine)
Gastrointestinal Tract Development (UNSW Embryology; Dr. Mark Hill)
GI Tract Development Snapshot Infographic (Cell Press)
Animation and narration of GI Tract development (University of Michigan):