Welcome to HCM,
will be prescribed for a high risk pregnancy
, ie which has previous history of abortion
Primolut injections 500mg contains norethisterone
which is a progestin which is normally used as contraception
, irregular menses bleeding etc.
There is no rationality for prescribing Primoult injections for maintenance of pregnancy since there is better alternatives of progesterone.
Vaginal pessaries may contain progesterone which is also enough for maintaining pregnancy.
But usually progesterone is enough for first 10 weeks of pregnancy later your placenta takes over progesterone production in body.
Get a good advice from your Gync regarding future treatment with drugs and get a USG scan done.