Q. Why does pregnancy sickness vary from one pregnancy to the next?
To cut a long story short, the pregnancy hormone, human chorionic gonadotrophin (hCG) has been associated with nausea and vomiting in pregnancy (N.V.P) by investigations, which include a number of women. However, there is one rare condition which is associated with high hCG blood levels, but is not associated with any sickness. As N.V.P in early pregnancy is related to rapidly rising maternal hCG blood levels, we need to look for the cause of N.V.P at other locally acting hormones which stimulate the synthesis (production) of hCG. There are, at our present state of knowledge, eleven locally acting hormones or cytokines all produced by placental (often called afterbirth) cells, which stimulate hCG synthesis. There are also at least 5 known variations of hCG itself. The only one of these locally acting hormones, which is known to cause sickness, is a group called Prostaglandins, of which Prostaglandin E2 (PGE2) and Prostaglandin F2 alpha (PGF2a) are constantly present during early pregnancy. There is an enzyme produced in some placental cells called Prostaglandin dehydrogenase (PGDH), which breaks down or inactivates these prostaglandins. The amount of these anti-prostaglandins in placental cells varies from one pregnancy to another and also increases in quantity as the pregnancy progresses.
There are then many hormones produced by the placenta that can vary in quantity from one pregnancy to another and be associated with different degrees of severity of N.V.P in each individual pregnancy. More research and more funding is needed for further progress to be made to explain clearly this surprising and important subject. We may notice that N.V.P recurs similarly from one pregnancy to the next in about ½ - ¾ of succeeding pregnancies in the same person.