RH factor and pregnancy: what you need to know

Hey everyone!
Let's lay this out:
The guy (Person A) is O negative.
The woman (Person B) is A positive.

So, we're looking at different Rh factors here. Does that mean it's harder for her to get pregnant, or that she'll need those shots during the pregnancy?
And seriously, how much does this actually tank the chances of conceiving?

thanks!

It doesn't mean

If it were actually the other way around, women would be getting shots after every single birth or miscarriage just to manage antibody levels.

I’m betting that if the kid turns out positive, they're getting an injection right at birth, if I recall correctly.

Basically, if you're an Rh-negative mom, you'll be getting that RhoGAM shot after giving birth or having a miscarriage...

Scott Allen10 said:An Rh-negative mother gets the RhoGAM shot following a birth or a miscarriage...

And what happens when the babies catch a cold?

@ OP - this has absolutely nothing to do with getting pregnant 👍
The only time this actually matters is once the baby is already on its way, and even then, it’s only an issue if the mom is Rh-negative and the baby is Rh-positive—and specifically when that exact combo happens for the third time in a row (whether you're talking about two previous miscarriages or two live births). That’s when the third kid runs into trouble.
To make it easy to wrap your head around: being Rh-positive means you have that specific marker, while Rh-negative means you don't. So, if a mom is Rh-negative and the baby is Rh-positive, during the pregnancy (via blood mixing), her body starts producing these random antibodies against that Rh-positive factor because her immune system flags it as a foreign invader. Basically, her body tries to fight its own baby's blood... In the first couple of pregnancies, the antibody count isn't high enough to actually hurt the baby, but by the third one in a row, it becomes a real problem. That's why an Rh-negative mom needs that injection after every birth or miscarriage; it essentially tricks her body into treating every subsequent pregnancy like it's the very first one.

Scott Allen10 said:An Rh-negative mother gets that "horn shot" injection after giving birth or a miscarriage...

@Scott Allen10, I've always wondered if you happen to know—why doesn't this same thing happen when the mom is Type O and the baby is Type A, B, or AB? Logically, it seems identical since it's still a marker the mom lacks but the baby has. Why isn't there the same reaction? We know from blood transfusions that you can't receive blood that has "add-ons" your own body doesn't recognize, whether that's the Rh factor or those specific blood type letters.
I've been curious about this since I was a kid, and nobody ever gave me a straight answer. 🤷

Angela Wright said:And what about when the babies get their shots?

As far as I know, there's no reason for the babies to get them, because it's only the mom's body that could potentially harm a third child. The baby isn't doing anything to itself (and again, this only applies if the previous two pregnancies were specifically that mom negative / baby positive combo).

wearygardener19 said:@Scott Allen10/">@@Scott Allen10, I’ve actually wondered about this forever—maybe you have some insight—why doesn't this exact same thing happen when the mom is type O and the baby is A, B, or AB? Logically, it feels identical since we're talking about a marker the mom lacks but the baby has. Why isn't there the same reaction? It seems like how you can't receive blood during a transfusion if it has an "extra" component you don't have, whether that's the Rh factor or one of those ABO letters.
I've been curious about this since I was a kid, and nobody ever really gave me a straight answer. 🤷

Man, I totally blanked on replying to this, so here it is a little late...

The truth is, that exact situation definitely happens, and we've seen hemolytic reactions and all sorts of complications from ABO incompatibility between a mother and her baby.

It's honestly pretty tough to give a simple answer because there's so much deep immunology involved... but let me try to break it down simply...

With Rh(D) immunization, the body produces IgG antibodies, which are tiny enough to slip right through the placental barrier without any trouble... but with ABO immunization, the body makes IgM antibodies, which are way too big to cross that barrier easily, so it's super rare for them to get through...
That doesn't mean it's impossible, though. The most critical moment for immunization usually happens during delivery, specifically when the placenta separates from the uterine wall. That's why the first pregnancy is generally safe—the mom hasn't been sensitized yet, so she doesn't have the antibodies ready to go. But if a mom has already been immunized and then has a second baby with an incompatible blood type, a hemolytic reaction—and a really serious one at that—is absolutely possible.
There's a whole lot more to it, but I feel like I'd be getting way too bogged down in the weeds if I went into every single detail... Basically, just like with the Rh(D) system, immunization and hemolytic reactions are possible in the ABO system too, it's just that for a few biological reasons, it doesn't happen nearly as often...

thanks, thanks for the reply 🙏
man, forget biology professors—honestly, even a handful of doctors won't help—it’s here where I finally find the answers to what's been bugging me forever 🙏

I did a little digging online and stumbled upon this bit you posted (they didn't mention those rare cases you were talking about)
The IgM molecule is a pentamer, which means its massive molecular mass (900,000) prevents it from leaking into extravascular space. B lymphocytes secrete M-class immunoglobulins as the primary response to an antigen. IgM doesn't cross the placenta, and it starts being synthesized five days after birth.
found this right here: http://kbsm.netfirms.com/klinkemija/...#_Toc500661908

pretty fascinating stuff 🙂
I noticed most places talk about IgM and IgG in terms of defending against viruses and other invaders, but it caught my eye that IgM stays in the bloodstream only briefly, signaling a fresh reaction to something. If they aren't in the blood, it means the infection is older (or wasn't there at all), but it also means there's no immunity left. In this specific scenario—a few months after birth—I'm guessing based on what I read that the IgM leaves the woman's body and fails to recognize whether the next baby is type A, B, or AB blood group. It's likely a danger to the baby if the mother gets pregnant too soon, like in those first few months postpartum...
whereas IgG antibodies stay in the system permanently, providing lasting immunity (so the body recognizes the threat immediately), which would mean it instantly identifies the baby's Rh+ status too.