Free-hcg during pregnancy should be “free β-HCG” referring to serum human chorionic gonadotropin free β-subunit, which is associated with the development of fetal chromosomal abnormalities and adverse pregnancy outcomes such as preterm labor, stillbirth, and fetal malformations. Abnormally high serum human chorionic gonadotropin free beta subunit in mid-pregnancy increases the risk of fetal chromosomal abnormalities, fetal distress, preeclampsia, placental abnormalities, preterm labor, and low birth quality. Fetal chromosomal abnormalities have the most serious impact on adverse pregnancy outcomes, most of which are Down’s syndrome, in which children are prone to die early in fetal life, and most of those who survive exhibit congenital deficiencies, such as low intelligence quotient (IQ) and developmental disorders. The detection of serum human chorionic gonadotropin free beta subunit in mid-pregnancy has high diagnostic value for fetal chromosomal abnormalities. HCG is secreted by placental trophoblast cells and is one of the triad of prenatal screening for Down syndrome, and studies have shown that this indicator is of great value for fetal Down syndrome, open neural tube, and chromosomal abnormalities.