According to one scientist's "semen cures morning sickness" theory, morning sickness might simply be a case of semen-deficiency.

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The theory was uncovered by Slate's Jesse Bering while exploring the baffling history of morning sickness: how doctors and researchers trying to figure out why pregnant ladies just can't keep food down sometimes, and how to stop it.

Basically, morning sickness is believed to be caused by a woman's body attempt to avoid pathogens. Since the body views the fetus as a foreign object, her immune system goes into "save baby" mode which makes the body more susceptible to food borne illness.  However, one SUNY-Albany psychologist named Gordon Gallup thinks its less about food and more about DNA:
To understand where [Gallup's] coming from, we need to think back to the maternal immune system's response to the fetus. Because half of the DNA the fetus is carrying comes from the father, the mother's body may initially treat the organism as foreign tissue or an infection. This response, Gallup says, triggers an immune reaction that is commonly experienced as nausea, vomiting, and malaise (aka morning sickness). The best cure for this type of sickness, says Gallup, is, strangely enough, the same thing as its cause.
According to Gallup, since the baby is theoretically comprised of a woman and her partner's foreign genetic material, one way to stop the vomiting is to make the nauseating object less foreign. And the best way he could come up with was to recommend these ladies to get up their sperm intake.

Thankfully, this theory hasn't been tested out yet. But if Gallup's theory was correct, wouldn't that mean mothers who become pregnant via artificial insemination should theoretically be more likely to experience morning sickness? And that devout, practicing Catholic women who don't use any barrier or hormonal contraception should theoretically have morning sickness very rarely? So far, nobody's made that connection and Bering explains why:
This is because conception and childbirth historically meant that a woman foreclosed on any other reproductive opportunities for 2 to 4 years, so pregnancies in which paternal investment was improbable would have meant an enormous gamble. Today, however, technological innovations such as barrier contraceptives (condoms reduce a woman's exposure to semen that would otherwise become familiar) and artificial insemination mimic some ancestral conditions. The maternal immune system has no way to distinguish between, say, conception by in vitro fertilization and rape.
 So unless you're into this sort of stuff, we wouldn't recommend it as a cure.