Sorry to know that you are wandering around somewhat lost in this rh-factor/RhoGam maze! It can indeed be confusing for anyone who questions conventional 'wisdom' on the matter...I'll try to break it down a bit. This is long, but I hope clarifying for you.
First, it IS known that on the whole, RhoGam has been helpful to most women/babies who use it--we now have far fewer complications/deaths from rh sensitization than before RhoGam was invented.
However, it's also known that it doesn't work for everyone, as one pp has pointed out. And it's known that while RhoGam AFTER birth has been very very helpful, RhoGam administered routinely during pregnancy has not reduced sensitization complications nearly as much as predicted when it was decided to administer it during pregnancy. We still just don't know enough, in spite of the med system's belief in prenatal RhoGam.
So I'm saying that even though most providers do push the prenatal RhoGam, there is not nearly the evidence of it's efficacy as providers like to believe--due to your research, naturally you are a little confused or doubtful. Plus there is the debate over troubles caused by thimerisol (in the past) and by ANY use of blood products (and all RhoGam is still made from blood products).
RhoGam is administered at 28wks because pregnancy is '40wks' or so; it is believed that RhoGam is effective for '12wks or so'. It is also understood that late pregnancy, and most especially during birth, is the most likely time-frame for blood mixing/sensitization to occur.
Have you had antibody screens during this pregnancy? Standard care for rh- moms should include antibody ('titer') screening of your blood every few weeks or so--well, or AT LEAST just before receiving prenatal RhoGam. If you are not carrying any antibodies, then you know you are not sensitized. Of course, if you've had RhoGam in the past 12 or more weeks, then you could test positive for antibodies due to the antibodies in the RhoGam that are now in your bloodstream. Let's say you got a prenatal dose at 16 wks. When it's time for the 28wk dose, you might still have antibodies in your bloodstream from the first dose--so a 'positive antibody screen' at 28wks, before the 2nd shot, would be assumed to be 'positive' due to the first shot.
Yes, the RhoGam antibodies will cross the placenta and can kill some of your babies blood cells. However, prenatal RhoGam is considered 'safe' because the amount of antibodies in a 'standard dose' of RhoGam is considered low enough that not much damage will be done to your babies blood supply--not enough baby blood-cells will be killed to cause your baby any harm. A healthy baby can compensate, can make more blood cells.
However--that standard dose of Rhogam is calculated to 'cover for' blood mixing of about 30 milligrams of your baby's blood mixing with yours. Let's say you got the prenatal shot, then you did suffer some disruption of the placenta during pregnancy or birth. If the amount of baby blood that got into your blood stream was MORE THAN 30 milligrams, then the standard dose of RhoGam would NOT be enough to prevent sensitization.
The way to be sure that the amount of RhoGam you receive is adequate to the task, is to have certain blood tests to actually measure how much baby blood is in your bloodstream. If there is more than 30 milligrams of baby blood present in your bloodstream, then you can get a higher dose of RhoGam--a dose appropriate for the amount of baby blood present. This is only possible IF you know there has been an event that could cause blood mixing/placental disruption: say you had a fall, or a car accident, or some bleeding during pregnancy for unknown reasons--these things can be reason to suspect placental disruption and would be reason to check your blood for the presence of baby's blood cells in your bloodstream.
2 things about this:
1. If you are receiving the 'routine' prenatal RhoGam, you are receiving it because it is already known that you DON'T have blood mixing. At least, providers are *supposed to* give you an antibody screen *before* giving RhoGam--because RhoGam doesn't work if you're already sensitized (and if you're sensitized already, then the RhoGam will just put MORE antibodies in your blood that can harm the baby). Anyway--so the prenatal dose is calculated to cover for a 'potential' blood mixing of about 30 ml. But if, during late pregnancy or birth there is blood mixing of MORE THAN 30ml, then the prenatal shot will NOT cover you. You can end up sensitized anyway.
2. You wouldn't want a higher dose of 'routine RhoGam' prenatally, because you would be crossing that line of assumed safety for baby. A higher dose means more antibodies in your bloodstream, potentially harming your baby. Now, if you DID have an event, and you did get the blood tests to show that your bloodstream was carrying more than 30ml of baby blood cells, then it would be considered safer to have that higher dose of RhoGam, than to ignore it or only get the standard dose.
Let's say there is an event of some kind during your pregnancy, and you are tested and found to be carrying 90ml of baby blood cells in your bloodstream. So, you get a triple dose of RhoGam. This MAY be somewhat of a burden for the baby you are carrying now--but with today's medical technology, it would be thought that most babies could fully survive blood cell destruction caused by the RhoGam. And it would be believed that by getting that triple dose you would be protecting all FUTURE babies--because the RhoGam would prevent sensitization--would prevent your immune system from starting to create it's own antibodies. Because once you start creating antibodies, you will continue to do so over time, it is a 'permanent' condition.
I'm saying that there are no guarantees, no 'absolutes'--but there is a mechanism for 'risk-benefit analysis' in the application of various tests, and the use of RhoGam. Well, that analysis can be useful (if not absolutely helpful to all) IF it is properly put in place. Unfortunately, it is not always properly used. From my own research, I believe that all women should be given the blood tests that actually measure for baby's blood cells in mom's bloodstream, before being given RhoGam after birth. But in most cases, providers go by 'standard protocols' and do not get those tests done. The standard protocol is to simply administer the standard dose of RhoGam--this is cheaper and easier than giving all Rh- moms expensive tests to measure for baby blood cells. Usually, the only time a mom would get those tests is if the provider knew of something during birth that was likely to cause blood mixing--say, placental abruption during labor, or a retained placenta that needed medical extraction.
If those tests were administered to all postpartum rh- moms, then many of them would be shown NOT TO NEED RHOGAM at all--because in most cases, blood mixing does not occur during birth! And for those who did have blood mixing during birth, then those tests would show HOW MUCH blood mixing occurred, and so, how much RhoGam to give the mom.
Anyway--it is my experience that many providers simply follow the protocols of x number of shots given during pregnancy, and everyone gets a shot after birth. They really don't know more in-depth info. Instead of properly applying risk-benefit analysis, it's more about 'cost-benefit analysis'--those thorough blood tests cost more $$, which has to be justified to insurance companies....follow the money.
But you can insist on those tests, yourself.
If I were having a hospital birth, I'd probably get the prenatal shot, because (generally speaking), the way hospitals handle birth and especially placental delivery puts you at higher risk for placental disruption and sensitization. Now, things can happen at home, too--and even some homebirth mws have methods that may increase risk--but I personally would feel the risk was greater in the hospital. Wherever I gave birth, I'd insist on getting postpartum blood tests to see whether or not I was carrying baby blood cells. If not, I'd refuse the postpartum RhoGam. If so, I'd want to be sure to get the right dose of RhoGam.
Most of my clients do NOT get the prenatal shot. None so far have been sensitized. Most do get the postpartum shot, 'just in case'. I don't tell them what to do; I give information and let them make up their own minds. I do urge moms to get those blood tests after birth before deciding whether or not to have the postpartum shot.
Hope this helps! Remember that with birth as with the rest of life, there are no guarantees. We can only make the decisions that seem best to ourselves, with the info and intuitions we have on a subject. Trust yourself, try not to worry about this. You will know what to do.