Why does one vanish?
Often the lost embryo has chromosomal issues that make it non-viable. The body detects this very early & reabsorbs the tissue. Chorionicity:- Separate placentas = much safer for the survivors. Shared placenta = higher monitoring needed if loss happens later.
This is vanishing twin (or triplet) syndrome. One embryo stops developing and is reabsorbed by the body. It usually happens in the first trimester, most often between 6–12 weeks. If it occurs before the first ultrasound (around 5–7 weeks), many parents never know multiples were present. Mild spotting or cramping can occur but often mimics normal early pregnancy symptoms.