Serum biochemistry
- In pregnancies with fetal chromosomal defects there are changes in the concentration of various placental products found in maternal blood
- The two placental products that can be used to calculate the risk for chromosomal defects at 11-13 weeks are free ß-hCG and PAPP-A
- These products must be measured with high quality instruments to give accurate results
- In addition, the measurements can be altered by various maternal characteristics (weight, ethnic origin, smoking), the number of fetuses and the method of conception (natural or by in vitro fertilization) and these factors must be taken into account for the calculation of risks