Just a FYI they will leave you with bruises. X
I've been on a blood thinner since 7 weeks pregnant and was given them on the day If you've been told from 28 weeks then maybe contact your dovecote and see if the midwife has said anything to them?
I meant to have injections too, as family history but I had my blood taken 2 times for them to make sure and discussed with the hematology team and they decided I don't need to take it just if I have a c section. I am lucky as my baby is an IVF baby and I had many injections up to 12 weeks.
They should contact you. I had them from 30 weeks, and they'll tell you how to inject yourself. I won't lie they aren't pleasant. I couldn't do my own and had my OH do them for me. X