Blastocyst transfer and monozygotic twinning.

OBJECTIVE: To report two cases of monozygotic twinning after IVF-blastocyst transfer.
DESIGN: Case report.
SETTING: Private practice in an assisted reproductive technology clinic. PATIENT(S): Two women treated with IVF-ET at the blastocyst stage. INTERVENTION(S): Pituitary down-regulation with luteal leuprolide acetate, ovulation induction with gonadotropins, IVF, sequential culture, blastocyst transfer, and P for luteal support. MAIN OUTCOME MEASURE(S): Levels of hCG, pelvic ultrasound examination, amniocentesis, obstetric follow-up, and cesarean section. RESULT(S): Two intrauterine monozygotic twin pregnancies occurred after IVF and blastocyst transfer. One of them was complicated by fetus-to-fetus transfusion syndrome and was delivered preterm by cesarean section; the other woman had a normal pregnancy and vaginal delivery. CONCLUSION(S): Monozygotic multiple gestations may be increased in IVF blastocyst transfers. The potential obstetric complications of this type of pregnancy should be discussed with patients.