BACKGROUND: Normal fertilization is usually considered to have occurred whe
n two pronuclei (2PN) and two polar bodies are observed. Exceptions are the
single pronucleated zygote resulting from asynchronous pronuclei.
CASE: A 29-year-old woman entered a program of intracytoplasmic sperm injec
tion and embryo transfer because of her husband's oligoasthenoteratozoosper
mia. Two cleavage-stage embryos (four blastomeres, grade 1 and 2) were obta
ined from one fertilized oocyte containing distinct 2PN and the other a sin
gle pronucleus (1PN). At 15 weeks' gestation the patient developed severe p
reeclampsia requiring termination of the pregnancy. Histopathologic examina
tion and DNA ploidy by image analysis were consistent with a twin pregnancy
combining a complete hydatidiform mole and normal pregnancy.
CONCLUSION: We hypothesize that this 1PN was at the origin of the hydatidif
orm mole. This case highlights the danger of transferring an embryo having
1PN.