The coat hanger isn’t sterile. It isn’t even clean.
If the woman, or girl, is alone she thrusts it blindly upwards into the vagina. She’s hoping it will get into her uterus and do something. She may or may not know that to get into the uterus the coat hanger has to navigate the small opening in the cervix called the os.
A coat hanger is technically narrow enough to get through a pregnant cervical os, but the end is sharp not tapered so it can lacerate and perforate. Getting any instrument through the cervix safely also requires visualization and knowledge of the correct amount of force.
If she’s lucky enough to get the coat hanger through her cervix it could easily sail right through the back or side walls of the uterus. The uterine wall is soft and easily perforated with the wrong instrument or unskilled hands. If the uterus is perforated on one of the sides there is a high risk of lacerating a uterine artery, as that is where they are located. If this happens a woman who is by herself could easily bleed to death before she gets appropriate medical care. These arteries pump a lot of blood.
The other danger with uterine perforation is the bowel. Puncturing bowel will hurt, but depending on her level of fear it might only be enough to cry out but not to ask for help. However, within the next 3 days the bowel perforation will most certainly kill her unless she gets appropriate medical care. That care will likely involve major surgery to drain abscesses, remove necrotic bowel, and possibly even a colostomy. The uterus will also be infected and may be damaged beyond repair.
If she was lucky and got that rough end of the coat hanger in and out of her cervix without puncturing something it is unlikely she will induce an abortion immediately. In this scenario the coat hanger is really just a vector for introducing infection. In 2-3 days or so she will cramp, and if fortunate her uterus will contract and she will pass the tissue at home. However, the bacteria from septic abortions often disseminates and each hour the condition remains untreated death takes a step closer. If she gets to the hospital in time and they can empty her uterus without killing her and she doesn’t have a resistant bacteria and she isn’t in septic shock and is otherwise healthy she will survive. That’s a lot of variables.
The coat hanger might miss the cervix altogether and puncture the top of the vagina. It could also hit the uterine arteries from this angle and likewise still devastate the bowel.
If the coat hanger is used by someone else, they may or may not get it through the cervix. Same risks of perforation and laceration, because a skilled operator would never use a coat hanger. However, not everyone would know that and some are too desperate to care. The medical sequelae are therefore all essentially the same, with one exception. The type of people who offer coat hanger abortions may also sexually assault their victim, because really, who’s going to report them to the police?
If you are going to joke about coat hangers and abortion you should know exactly what you are joking about.