Medical Causes of Morning Sickness
Although scientists have not isolated the exact cause and mechanisms of morning sickness (NVP), researchers agree that it corresponds to the dramatic increase of two hormones during early pregnancy – Progesterone and hCG (human chorionic gonadotropin). Each plays a role in healthy embryo development, but also affects the biochemistry of the mother’s body.
The most widely accepted theory, researched and supported by Goodwin (2000) and others, shows that levels of hCG dramatically rise, peak, then diminish from week 5 to week 15 of pregnancy. And that is the exact same time-frame as the rise, peak, and diminishing of most morning sickness.
So there is a strong correlation between levels of hCG hormones in the bloodstream and the arrival/departure of the symptoms of morning sickness. Progesterone is also seen as playing a role in bringing about morning sickness nausea. The dominant hormone during pregnancy, progesterone helps prevent early labor by relaxing the muscles in the uterus. But it can also have a “softening” effect on the stomach muscles, making the stomach especially sensitive to discomfort.
The Anatomy
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Bloodstream (Circulatory System)
Placenta
Surrounds growing embryo, produces hormone: hCG
Maternal Ovary
Produces hormones: Estrogen, Progesterone
Hormones (Endocrine System)
hCG (human chorionic gonadotropin) – hormone released during pregnancy
Progesterone – hormone that relaxes stomach muscles
Medulla (lower brain stem)
CTZ (Chemoreceptor Trigger Zone)
Vomiting Center
Vagus Nerve
sends signal to your stomach that produces discomfort, then nausea
Stomach
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The Physiology
According to this hormonal-cause theory, the physiology of morning sickness would look like this:
Starting early in pregnancy, the hormone hCG is produced in rapidly increasing levels by the placenta surrounding the fetus. And the hormone progesterone is produced in increasing levels by the maternal ovary. These hormones, now highly concentrated, circulate in the bloodstream of the mother and reach the brain and stomach.
In the stomach, the higher levels of progesterone in the bloodstream relax the muscles and slow down digestion, causing the stomach to become more sensitive and easier to upset.
In the brain, the Chemoreceptor Trigger Zone, located in the Medulla (lower brainstem), is responsible for monitoring the bloodstream for toxins, drugs, chemicals, and other stimulants that might be harmful to the body. As a mechanism for protecting the body from harm, the Chemoreceptor Trigger Zone can send signals to the Vomiting Center of the brain (also located in the Medulla). This serves to empty the contents of the stomach in case that’s a source of contamination.
The high levels of hCG in the bloodstream activate the CTZ (Chemoreceptor Trigger Zone) either directly or indirectly. As a result, the CTZ sends neurological signals to the Vomiting Center of the brain which takes this and other information to decide whether or not to send sickness-inducing signals through the Vagus Nerve to the stomach. – The result can be nausea and vomiting, the classic symptoms of “Morning Sickness”.
Academic Citations:
Goodwin TM “ Human chorionic gonadotropin and hyperemesis gravidarum”, in Nausea and Vomiting of Pregnancy: State of the Art 2000, Gideon Koren (ed.) Motherisk Press © 2000 |
