What Causes Fetal Hiccups?

 by R. Y. Langham, Ph.D.

Fetal hiccups begin early in pregnancy, around the eighth to tenth week of gestation. When hiccups begin, the baby is too small for the mother to notice them. But as the baby grows, many women will feel them.

Fetal hiccups begin early in pregnancy, around the eighth to tenth week of gestation. When hiccups begin, the baby is too small for the mother to notice them. But as the baby grows, many women will feel them. When a fetus hiccups, the mother notices spasms in her abdomen that are different from other pregnancy movements. Fetal hiccups are a normal part of a baby's development. Occasionally they may indicate that there is a problem with the umbilical cord.

Diaphragm Spasms

Like hiccups in children and adults, hiccups in the fetus are caused by sudden strong contractions, or spasms, of the diaphragm -- the muscle that separates the lungs from the abdomen. Shortly after the diaphragm contracts, the vocal cords move together, closing off the windpipe that leads to the lungs. But hiccups in the fetus are different from hiccups after birth. When researchers use instruments to listen for fetal sounds, fetal hiccups do not produce any sound. The fetus' mouth also does not open during hiccups. Fetal hiccups generally appear as a series of many, repeated hiccups -- up to 60 hiccups per minute, according to a chapter in "Development of Normal Fetal Movements: The First 25 Weeks of Gestation."

Preparation for Breathing

Fetal hiccups are reflex movements that occur naturally, but their exact origin and function are unknown. They do not seem to be the result of an overfull stomach, which is a major reason for hiccups in children and adults. One leading theory is that fetal hiccups occur as a way to help strengthen the diaphragm so it works better when the fetus starts to make breathing movements. Although a baby doesn't actually breathe while still in the womb, it does make breathing movements in preparation for its life after birth. As the diaphragm is the main muscle responsible for breathing, having a strong diaphragm is important. This preparation for breathing movements theory is supported by the fact that fetal hiccups are most common at 10 to 12 weeks gestation. After this, they become less common as fetal breathing movements become more frequent.

Other Theories

Alternatively, fetal hiccups may simply reflect man's origin from animals and have no function at all in human fetuses. This evolution theory suggests that hiccups are just a carryover from animals for which hiccups have an important function. Tadpoles, for example, constantly swallow water, and windpipe closure during hiccups is necessary to prevent water from entering their lungs.

Another theory states that hiccups may help the fetus develop reflexes that close the windpipe and prevent substances from entering their lungs after they are born. These reflexes are important, as they allow the newborn to be fed without choking. One problem with this theory is that it fails to explain why hiccups becomes less frequent as the fetus gets closer to birth.

Umbilical Cord Compression

Occasionally fetal hiccups may indicate that the fetus is not getting enough oxygen and nutrients. When the umbilical cord becomes narrowed, or compressed, blood flow from the placenta to the baby decreases. Because the placenta is the fetus' source of oxygen and nutrients, this causes the fetus to be deprived of these vital substances. Sensing this, reflexes in the fetus may increase the number of body movements, including hiccups.

The author of an August 2012 article in "BMC Pregnancy and Childbirth" stated that women who notice fetal hiccups on a daily basis after 28 weeks gestation, and more than 4 times per day, should see their doctor to check for possible umbilical cord compression. A study published in the February 2017 issue of "The Journal of Maternal-Fetal and Neonatal Medicine," however, reported that fetal hiccups may occur more frequently without indicating any problems. In that study, women with normal pregnancies felt hiccups about 5 times per day on average at 28 to 33 weeks gestation and about 4 times per day on average at 34 to 39 weeks. As these were averages, hiccups were more common in many of these women.

Reviewed by: Mary D. Daley, MD


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