Risk factors for monozygotic twinning after in vitro fertilization: a systematic review and meta-analysis.

OBJECTIVES: To establish the risk factors for monozygotic twin (MZT) and monochorionic twin (MCT) pregnancies after in vitro fertilization (IVF).

DESIGN: Systematic review and meta-analysis.

SETTING: Not applicable.

PATIENT(S): Women who achieved MZT and non-MZT pregnancies through IVF.

INTERVENTION(S): Systematic search of Medline from January 1995 to October 2018 with cross-checking of references from relevant articles in English.

MAIN OUTCOME MEASURE(S): Possible risk factors for MZT or MCT pregnancies after IVF, comprising extended embryo culture, insemination method (conventional IVF and intracytoplasmic sperm injection [ICSI]), embryo biopsy for preimplantation genetic testing for aneuploidies or for monogenic/single-gene defects (PGT-A or PGT-M) programs, assisted hatching (AH), oocytes donation, female age, and embryo cryopreservation.

RESULT(S): A total of 40 studies were included. Blastocyst transfer compared with cleavage-stage embryo transfer, and female age <35 years were associated with a statistically significant increase in the MZT and MCT pregnancy rate after IVF: (23 studies, OR 2.16, 95% CI, 1.74-2.68, I2 =78%; 4 studies, OR 1.29; 95% CI, 1.03-1.62, I2 =62%; and 3 studies, OR 1.90, 95% CI, 1.21-2.98, I2 =59%; 2 studies, OR 2.34; 95% CI, 1.69-3.23, I2 =0, respectively). Conventional IVF compared with ICSI and assisted hatching were associated with a statistically significantly increased risk of MZT pregnancy (9 studies, OR 1.19, 95% CI, 1.04-1.35, I2 =0; 16 studies, OR 1.17, 95% CI, 1.09-1.27, I2 =29%, respectively). Embryo biopsy for PGT-A or PGT-M, embryo cryopreservation, and oocytes donation were not associated with MZT pregnancies after IVF.

CONCLUSION(S): Blastocyst transfer is associated with an increased risk of both MZT and MCT pregnancies after IVF. Further evidence is needed to clarify the impact of female age, insemination method and AH on the investigated outcomes.