Related Experiment Video
Updated: Aug 23, 2026

Human Blastocyst Biopsy and Vitrification
Published on: July 26, 2019
Monozygotic twinning associated with day 5 embryo transfer in pregnancies conceived after IVF
Victoria Wright1, Laura A Schieve, Anjel Vahratian
1Division of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention, Atlanta, GA, USA. vwright@cdc.gov
Background:
This study examines the association between day of embryo transfer and monozygotic (MZ) twinning.
Methods:
We used a population-based sample of 108,36 IVF/embryo transfer procedures in which the patients oocytes' were freshly fertilized (non-frozen; non-donor) and 39,98 resultant pregnancies from US clinics in 1999 and 2000. Cases were pregnancies for which the number of fetal hearts observed on ultrasound exceeded the number of embryos transferred. These pregnancies were considered to contain at least one set of MZ twins. A total of 226 MZ pregnancies were compared with two control groups: 23,880 singleton pregnancies (one fetal heart) and 15,092 other multiple-gestation pregnancies (> or = 2 fetal hearts but the number of fetal hearts on ultrasound was less than or equal to the number of embryos transferred).
Results:
Cases of presumed MZ multiple-gestation pregnancies were more likely to have had a day 5 embryo transfer compared with day 3 embryo transfers than singleton pregnancies [adjusted odds ratio (AOR) = 3.92, 95% confidence interval (CI) = 2.97-5.17] or other multiple-gestation pregnancies (AOR = 3.91, 95% CI = 2.96-5.17) conceived with IVF/embryo transfer.
Conclusions:
Day 5 embryo transfer may be associated with increased MZ twinning.
Related Concept Videos
In Vitro Fertilization
The IVF process begins with ovarian stimulation, during which reproductive endocrinologists prescribe hormonal medications to stimulate the ovaries to produce multiple eggs instead of the single...
Fertilization
Cleavage and Blastulation
Meiosis II
Ovarian Cycle
Infertility in Females
Endometriosis, a condition characterized by abnormal growth of endometrial...

