However, in women pregnant with twins rather than a singlet, the elimination half-life of OHPC was found to be shorter than this, at 10 days. OHPC has been detected in pregnant women up to 44 days after the last dose. Elimination
OHPC is eliminated 50% in feces and 30% in urine when given by intramuscular injection to pregnant women. Both the free steroid and conjugates are excreted by these routes, with the conjugates more prominent in feces. Veterinary
The pharmacokinetics of OHPC in various ungulates including cattle, buffalo, sheep, and goat have been studied. Time–concentration curves
Chemistry
OHPC, also known as 17α-hydroxyprogesterone caproate or as 17α-hydroxypregn-4-ene-3,20-dione 17α-hexanoate, is a synthetic pregnane steroid and a derivative of progesterone. It is specifically a derivative of 17α-hydroxyprogesterone with a hexanoate (caproate) ester at the C17α position. Analogues of OHPC include other 17α-hydroxyprogesterone derivatives such as algestone acetophenide (dihydroxyprogesterone acetophenide), chlormadinone acetate, cyproterone acetate, hydroxyprogesterone acetate, hydroxyprogesterone heptanoate, medroxyprogesterone acetate, and megestrol acetate, as well as the caproate esters chlormadinone caproate, gestonorone caproate (norhydroxyprogesterone caproate), medroxyprogesterone caproate, megestrol caproate, and methenmadinone caproate. Synthesis
Chemical syntheses of OHPC have been described. : 6

History
Along with hydroxyprogesterone acetate, OHPC was developed by Karl Junkmann of Schering AG in 1953 and was first reported by him in the medical literature in 1954. It was reportedly first marketed in Japan in 1954 or 1955, and was subsequently introduced as Delalutin in the United States in 1956. Due to its much longer duration than parenteral progesterone, OHPC had largely replaced progesterone in clinical practice by 1975.
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.