Triploid syndrome, also called triploidy, is a chromosomal disorder in which a fetus has three copies of every chromosome instead of the normal two. If this occurs in only some cells, it is called mosaic triploidy and is less severe.

Most embryos with triploidy miscarry early in development.

Many organ systems are affected by triploidy, but the central nervous system and skeleton are the most severely affected:

Common central nervous system defects seen in triploidy include holoprosencephaly, hydrocephalus (increased amount of cerebrospinal fluid within the brain), ventriculomegaly, Arnold–Chiari malformation, agenesis of the corpus callosum and neural tube defects.^{[citation needed]}

Skeletal manifestations include cleft lip/palate, hypertelorism, micrognathia, club foot and syndactyly/^{[citation needed]}.

Congenital heart defects, hydronephrosis, omphalocele, microphthalmia, macroglossia and meningocele (spina bifida) are also common. Cystic hygromas occur but are uncommon.^{[citation needed]}

Triploid fetuses have intrauterine growth restriction beginning early in the pregnancy, as early as 12 weeks, and does not affect the head as severely as the body. Oligohydramnios, low levels of amniotic fluid, is common in triploid pregnancies. Placental abnormalities are common in triploidy. Most frequently, the placenta is enlarged and may have cysts within. In some cases, the placenta may be unusually small, having ceased to grow.

During the first trimester, fetuses with triploidy have a thicker fluid under the skin behind their neck, which calls for observation during the first and second trimester of gestation.^{[citation needed]}