How does αlpha make adjustments for in-vitro fertilization pregnancies?

This general question often relates to several specific questions:

1. Why are adjustment factors used and what are they?

Serum marker levels may differ, on average, between women who undergo in-vitro fertilisation (IVF) and those who conceive naturally.  If this is not taken into account, it can lead to differences in the false-positive rate between these two groups.  To avoid this, αlpha uses adjustment factors when interpreting reports for women who have undergone IVF. The adjustment factors are the median marker MoM levels in IVF pregnancies (which are 0.94, 1.14 (Reference 1)  and 0.91 (Reference 2) in ue3, total hCG and PAPP-A respectively). αlpha divides the observed MoM values for an individual by the adjustment factors and these adjusted MoM values are used to calculate the risk estimate.

2. When a woman receives a donated egg, what maternal age is used in the risk calculation?

When eggs are donated from a third party, the donor’s date of birth is entered into αlpha .  αlpha uses the donor’s age at the expected date of delivery to calculate the maternal age related risk of the pregnancy being affected with Down’s syndrome.

3. When a frozen embryo is implanted should the date of egg collection or embryo implanatation be used to calculate the maternal age?

When a frozen embryo is implanted the date on which the eggs were collected (which could be several years prior to implantation) is entered into αlpha rather than the date of implantation. This date is used to determine the age of the mother or donor at the expected date of delivery and hence the maternal age related risk of the pregnancy being affected with Down’s syndrome.

4. How is the date of embryo transfer used to estimate gestational age?

The date on which an embryo is transferred into the mother is used to estimate gestational age. The gestational age is taken to be 18 days longer than the time since embryo transfer because the LMP date is about 18 days before the date of conception (Reference 3). If the date of embryo transfer is recorded, this takes precedence over other estimates of gestational age.

References

1.Serum markers for Down's syndrome in women who have had in vitro fertilisation: implications for antenatal screening (1999)
2.Adjustment of serum markers in first trimester screening (2009)
3.The estimation of gestational age of pregnancy for use in screening for Down’s syndrome using ultrasound measurements and embryo transfer date (2012)