Triploidy and monosomy X (Turner syndrome) are the two most common chromosomal anomalies at conception. Their prevalence decreases with increasing gestational age (GA), and they are relatively rare at birth. These genetic conditions can be prenatally or postnatally detected by karyotyping [chromosome analysis of fetal cells collected by chorionic villous sampling (CVS) or amniocentesis or blood lymphocytes]. Triploidy is caused by a complete extra set of chromosomes (the presence of 69 chromosomes). If the extra set of chromosomes is maternal in origin, the disease is called digynic triploidy, while if the extra set of chromosomes is paternal in origin, the disease is called diandric triploidy. Triploidy is lethal, most affected embryos are miscarried; however, some individuals survive a few hours or days after birth. Turner syndrome is an aneuploidy (monosomy X) in which only one chromosome X is present, and the other sex chromosome is either absent or abnormal. Severe forms of Turner syndrome present major fetal anomalies, including fetal hydrops and usually progress to early fetal demise (the lethal Turner phenotype). Postnatally, Turner syndrome presents as a form of primary ovarian insufficiency, determined by a gonadic digenesis (not by agenesis). Both triploidy and Turner syndrome must be taken in consideration if sonography reveals minor or major abnormalities in the first or second trimester of pregnancy. Active screening in the general pregnant population is not justified for these conditions alone. Both conditions can be diagnosed prenatally in individual cases.