Not herbs, but all natural...
I hope everyone is grownup enough to read this with a mature eye and if there are any young shoulder readers that the medical terminology makes it over their heads IYKWIM...
This should make the husbands/male partners/fathers/babydaddys in the crowd happy...

Anyhow, this is from Wikipedia and in the article here:
http://en.wikipedia.org/wiki/Pre-eclampsia are the links to the original studies.
Excerpt:"Research on the immunological basis for pre-eclampsia has indicated that continued exposure to a partner's semen has a strong protective effect against pre-eclampsia, largely due to the absorption of several immune modulating factors present in seminal fluid.[32] Studies also showed that long periods of sexual cohabitation with the same partner fathering a woman's child significantly decreased her chances of suffering pre-eclampsia.[16] Several other studies have since investigated the strongly decreased incidence of pre-eclampsia in women who had received blood transfusions from their partner and in women who had been regularly performing oral sex,[33] with one study concluding that "induction of allogeneic tolerance to the paternal HLA molecules of the fetus may be crucial. Data collected strongly suggests that exposure, and especially oral exposure to soluble HLA from semen can lead to transplantation tolerance."[33]
Other studies have investigated the roles of semen in the female reproductive tracts of mice, showing that "insemination elicits inflammatory changes in female reproductive tissues,"[34] concluding that the changes "likely lead to immunological priming to paternal antigens or influence pregnancy outcomes." A similar series of studies confirmed the importance of immune modulation in female mice through the absorption of specific immune factors in semen, including TGF-Beta, lack of which is also being investigated as a cause of miscarriage in women and infertility in men.
According to the theory, some cases of pre-eclampsia are caused by an abnormal maternal immune response to the fetus and placenta, which both contain "foreign" proteins from paternal genes, but regular exposure to the father's semen may promote implantation, a process which is significantly supported by as many as 93 currently identified immune regulating factors in seminal fluid.[6][14]
Having already noted the importance of a woman's immunological tolerance to her baby's paternal genes, several Dutch reproductive biologists decided to take their research a step further. Consistent with the fact that human immune systems tolerate things better when they enter the body via the mouth, the Dutch researchers conducted a series of studies that confirmed a surprisingly strong correlation between a diminished incidence of pre-eclampsia and a woman's practice of oral sex, and noted that the protective effects were strongest if she swallowed her partner's semen.[33][35] The researchers concluded that while any exposure to a partner's semen during sexual activity appears to decrease a woman's chances for the various immunological disorders that can occur during pregnancy, immunological tolerance could be most quickly established through oral introduction and gastrointestinal absorption of semen.[33][35] Recognizing that some of the studies potentially included the presence of confounding factors, such as the possibility that women who regularly perform oral sex and swallow semen also engage in more frequent intercourse, the researchers also noted that, either way, the data still supports the main theory behind all their studies--that repeated exposure to semen establishes the maternal immunological tolerance necessary for a safe and successful pregnancy.[35][36]
A team from the University of Adelaide has also investigated to see if men who have fathered pregnancies which have ended in miscarriage or pre-eclampsia had low seminal levels of critical immune modulating factors such as TGF-Beta. The team has found that certain men, dubbed "dangerous males," are several times more likely to father pregnancies that would end in either pre-eclampsia or miscarriage.[36] Among other things, most of the "dangerous males" seemed to lack sufficient levels of the seminal immune factors necessary to induce immunological tolerance in their partners.
If the theory of pre-eclampsia as a symptom of immune intolerance in some cases is officially accepted, women who suffer repeated pre-eclampsia, miscarriages, or In Vitro Fertilization failures could potentially be administered key immune factors such as TGF-beta along with the father's foreign proteins, possibly either orally, as a sublingual spray, or as a vaginal gel to be applied onto the vaginal wall before intercourse.[36]