Weak D and Pregnancy

[browser2025]

I’m wondering if anyone has any knowledge or experience with blood type and pregnancy, especially the “weak D” type. I’d always believed my blood type was O-negative as that’s what a previous medical test and two home kits showed. I know this matters in pregnancy because of the rhesus factor and potential sensitisation risks with any subsequent pregnancies.

When I was pregnant with DC1, my blood test came back inconclusive at first and had to be rechecked at the lab. The final result came back as “weak D,” which completely confused me. It was explained that it’s basically O-positive, but with very weak markers something shows up, but only faintly.

My midwife at the time didn’t seem entirely confident explaining it (understandably, she’s not a haematologist). I’ve done a lot of Googling since, but now that I’m pregnant again, I’m worried.

I am due to have my first midwife appointment and my blood test for this pregnancy, however if like last time the midwife has no knowledge of this, and I’m again unable to push for clarification from an expert, could it affect whether I’m given an anti-D (Rhogam) shot if I actually need it?

Has anyone else been told they have weak D or had a similar experience? (I appreciate there may not be). Should I trust the result or challenge it?

Thank you 🙏🏻

A weak D phenotype in pregnancy requires careful management to prevent Rh sensitization.
As a mother with a weak D antigen could still form antibodies against a Rh-positive fetus, leading to hemomytic disease og the fetus/newborn.

Management depends on the specific type of weak D and national guidelines, but traditionally, women with weak D phenotypes have been managed as RhD-negative, receiving RhIg injections.

However modern practices are moving towards using molecular testing to identify specific weak D types, which can sometimes avoid the need for RhIg and transfusion with RhD-negative blood if the type is not considered at risk of sensitization.

Tbh it's not a midwife but an obstetrician you need to see.

Hi OP, I've never heard of weak D - this can't be common?

But, regarding the injection,
I think they can give people the shot, even if they're not sure about blood type - ie I don't think it needs to only be used for people who are definitely rhesus nehative.

I only believe this anacdotally from my mum - who said that in the 90s after my sis was born the doctor didn't know her blood type and couldn't get any blood so just gave her the shot on the off chance - which was lucky because she was rhesus negative.
So as much as I'd push for knowing more about your blood type, I'd also push for the shot to be safe.

The percentage of the weak D blood type is between 0.2% and 1% of Caucasians. It's slightly more common in people of African American descent. It's less common in East Asian.

Thanks. I’m in the UK and routine antenatal care during pregnancy is primarily provided by midwives. But based on this I think I’ll try and push to see one instead as it’s definitely classed as a risk factor isn’t it. Thanks for the info.

Interesting. So I might be able to push for the shot anyway, just to be on the safe side. I’ll explore this. Thank you 🙏🏻