Final answer:
AFP levels are tested in maternal serum during the second trimester, between 16 and 18 weeks gestation, and are used to screen for fetal anomalies and certain cancers in survivors.
Step-by-step explanation:
The maternal serum is tested for AFP (alpha-fetoprotein) levels during the second trimester of pregnancy, typically between 16 and 18 weeks gestation. Alpha-fetoprotein is a protein that is normally produced by the fetus. High levels of AFP in the maternal serum can suggest certain conditions such as neural tube defects or abdominal wall defects, and low levels can be associated with chromosomal abnormalities, such as trisomy 21 (Down syndrome), especially when correlated with maternal age.
The incidence of having a fetus with trisomy 21 also increases dramatically with maternal age. It is vital to monitor AFP levels in certain cancer survivors, for example, an elevated AFP in a child previously treated for a teratoma may suggest a relapse with an endodermal sinus tumor.