Trend of medically induced monozygotic twin deliveries according to age, parity, and type of assisted reproductive technique during the period 2007–2017 in Lombardy Region, Northern Italy: a population-based study

PURPOSE: The risk of monozygotic (MZT) twinning is increased in pregnancies after assisted reproductive technologies (ART). However, determinants remain poorly understood. To shed more light on this issue, we analyzed the estimated frequency of MZT twins from ART in Lombardy, Northern Italy, during the period 2007–2017. METHODS: This is a population-based study using regional healthcare databases of Lombardy Region. After having detected the total number of deliveries of sex-concordant and sex-discordant twins from ART, we calculated MZT rate using Weinberg’s method. Standardized ratios (SRs) and corresponding 95% confidence intervals (CI) of MZT deliveries, adjusted for maternal age, were computed according to calendar period, parity, and type of ART. RESULTS: On the whole, 19,130 deliveries from ART were identified, of which 3,446 were twins. The estimated rate of MZT births among ART pregnancies was higher but decreased over time (p-value = 0.03); the SRs being 1.33 (95% CI: 1.18–1.51), 0.96 (95% CI: 0.83–1.11), and 0.92 (95% CI: 0.79–1.07) for the periods 2007–2010, 2011–2014, and 2015–2017, respectively. The SRs of MZT among women undergoing first-level techniques, conventional in vitro fertilization (IVF), and intracytoplasmic sperm injection (ICSI) were 0.47 (95% CI: 0.38–0.57), 1.02 (95% CI: 0.88–1.17), and 1.43 (95% CI: 1.27–1.61) (p-value < 0.0001). The ratio of MZT births was significantly higher in women younger than 35 years (p-value < 0.0001) and slightly higher among nulliparae (p-value < 0.0001). CONCLUSION: Despite a reduction of MZT rate from ART over the time, the risk remains higher among ART pregnancies rather than natural ones. Younger women and women undergoing ICSI showed the highest risk of all. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10815-021-02268-0.