When an Rh‑negative woman conceives with an Rh‑positive man, the chance of having an Rh‑positive fetus is usually 50% , but it can be as high as 100% depending on the father’s genes.

Core idea

  • The Rh factor is controlled by a gene with a dominant Rh‑positive allele (R) and a recessive Rh‑negative allele (r).
  • An Rh‑negative woman must have the genotype rrrrrr (no Rh protein on her red blood cells).
  • An Rh‑positive man can be:
    • RR (homozygous positive) → all children get an R from him and an r from her → all are Rr (Rh‑positive) → 100% Rh‑positive fetuses.
* **Rr (heterozygous)** → each child has a 50% chance of getting R (Rr, Rh‑positive) and 50% chance of getting r (rr, Rh‑negative) → **50% Rh‑positive fetuses**.

So, unless genetic testing shows the father is definitely RR, most teaching and exam-style questions assume he is heterozygous, and the answer given is 50% chance the fetus will be Rh‑positive.

Why this matters in pregnancy

  • Rh incompatibility only becomes an issue if:
    • Mother is Rh‑negative (rr) and
    • Fetus is Rh‑positive (Rr).
  • In that situation, fetal Rh‑positive cells that cross into the mother’s circulation can trigger her immune system to make antibodies against Rh‑positive blood, which can harm a future Rh‑positive fetus if not prevented.

Because of this risk, guidelines recommend routine Rh testing in pregnancy and, if needed, Rh immune globulin (RhoGAM) to prevent sensitization.

In exam or quick‑quiz wording like your question, the expected single best answer is: there is a 50% chance their fetus will be Rh‑positive.

TL;DR:

  • If dad is assumed heterozygous (common assumption) → 50% Rh‑positive fetus.
  • If dad is known homozygous Rh‑positive → 100% Rh‑positive fetus.

Information gathered from public medical and educational sources available on the internet and portrayed here.