There are two drugs they use to assist with the 3rd stage, namely syntometrine and syntocinon. The first one is the most common and this can sometimes cause the cervix to shut before the placenta is delivered. The 2nd one works more slowly and is not linked with a higher incidence of retained placenta. You could request, if you decide to have a managed 3rd stage, to have syntocinon instead of syntometrine. You also have the option of having no injection for 3rd stage at all (called a physiological 3rd stage) however this can sometimes result in a larger post birth bleed and you will possibly be considered high risk for a bleed anyway because bleeding often goes hand in hand with a retained placenta, so be prepared for a bit of a debate with your caregivers if you choose to go down that route.