Hyperemesis Gravidarum: Understanding Morning Sickness & Pregnancy

Question
I had horrible morning sickness with my first and only pregnancy with my son.  I have heard of women who have hyperemesis with one pregnancy, then do not have hyperemesis with other pregnancies.  I am familiar with the phrase, "every pregnancy is unique."  However, I am wondering if there is any scientific research about this correlation?
And what causes hyperemisis?  Is it a hormone sensitivity? Does the baby's gender play a role?  If you are taking IVF progesterone/estrogen for post-transfer pregnancy support, do the hormones increase risk of hyperemisis?

Answer
I don't think there's enough scientific research to answer your questions very well.  We don't know what causes hyperemesis.  It probably has something to do with HCG hormone, which gets very high during the first trimester and then gets much lower for the remainder of the pregnancy.  As far as we know, it's not a hormone sensitivity, since hyperemesis is more common in conditions where HCG is higher, such as twins and hydatidiform mole.  The baby's gender doesn't seem to play a role.  I don't think there is any relationship with the progesterone/estrogen from IVF.  Estrogen and progesterone levels get higher later on with normal pregnancies.  Women who have had hyperemesis have an increased chance (over other women) of having it with subsequent pregnancies, but many do not.  I wish we knew more, but we don't.