Vanishing triplet (one or more embryos resorbed in a triplet pregnancy) occurs in roughly 50% of cases that start with three gestational sacs. Natural triplet pregnancies are already rare at about 1 in 7,000–8,000. Relative to all worldwide pregnancies this is uncommon—on the order of 1 in several thousand to 10,000 overall, though rates rise with IVF/ART.
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.