These include:
Decreased myometrial activity with progesterone in vitro but no effect or increased myometrial activity with OHPC
Prevention of cervical ripening with progesterone but unknown effect with OHPC
A non-significantly increased rate of stillbirth and miscarriages with OHPC (in one study)
A possibly increased incidence of gestational diabetes with OHPC (increased in two studies, no difference in one study) but no such effect with progesterone
A significantly increased risk of perinatal adverse effects such as fetal loss and preterm delivery in multiple gestations with OHPC (in two studies)Differences in the metabolism of progesterone and OHPC and differences in the formation and activities of metabolites may be responsible for or involved in these observed biological and pharmacological differences. Progesterone is metabolized by 5α- and 5β-reductases, 3α- and 3β-hydroxysteroid dehydrogenases, and 20α- and 20β-hydroxysteroid dehydrogenase in various tissues. In target tissues, particularly the cervix and myometrium, these enzymes regulate local progesterone concentrations and can activate or inactivate progesterone signaling. In addition, these enzymes catalyze the formation of metabolites of progesterone such as 5β-dihydroprogesterone and allopregnanolone, which signal through their own non-genomic receptors such as membrane progesterone receptors and the GABAA receptor and have their own important effects in pregnancy. As examples, 5β-dihydroprogesterone has been found to play an important role in suppressing myometrial activity while allopregnanolone has potent sedative and anesthetic effects in the mother and especially the fetus and is involved in fetal nervous system development.
Of these women, 9 had infants with congenital malformations (2%), but there was no consistent pattern and none involved internal organs. OHPC is currently (as of June 2014) pregnancy category B, meaning there is no evidence of fetal risk with use of this medication during pregnancy. Although this is now the recommendation, this has not always been the case. A review by Marc Keirse of Flinders University concluded that information about the potential harms was lacking. Three clinical studies in singleton pregnancies of 250 mg/week of intramuscular OHPC have all shown a trend for an increase in pregnancy loss due to miscarriage compared to placebo. One of them, a large National Institutes of Health (NIH) study in 2003, looked at the effect of OHPC injections in women at risk for repeat premature birth and found that the treated group experienced premature birth in 37% versus 55% in the controls. A follow-up study of the offspring showed no evidence that OHPC affected the children in the first years of life. Based on these NIH data, OHPC was approved by the Food and Drug Administration (FDA) in 2011 as a medication to reduce the risk of premature birth in selected women at risk. (v.i.) The FDA expressed concern about miscarriage at the 2006 advisory committee meeting; the committee voted unanimously that further study was needed to evaluate the potential association of OHPC with increased risk of second trimester miscarriage and stillbirth.