title: Risk of Monozygotic Twins After Assisted Reproduction: A Population-Based Approach.
abstract: BACKGROUND
  Recent studies have suggested that ovarian stimulation and assisted reproductive techniques (ART) may increase the frequency of monozygotic twins. In this article, we present the analysis of the estimated frequency of twin deliveries following in vitro fertilization (IVF) in Lombardy during the period 2010-2014 for a total of 450,949 pregnancies.
  
  
  METHODS
  This is a population-based study using data from the regional data base of Lombardy, a northern Italian region with a population of about 10 million inhabitants. During the considered period, a total of 461,424 single or multiple births were registered in Lombardy. After exclusion of triplets or more pregnancies, the total number of twin deliveries, in separate strata of like and unlike sex pregnancies twin deliveries, were obtained and the rate of twin deliveries was computed according to spontaneous and non-spontaneous conception and type of ART. Further, estimates of dizygotic or monozygotic twin births were calculated using Weinberg's methods.
  
  
  RESULTS
  The frequency of twins deliveries was 1.24/100 deliveries after natural conception and 20.05 after assisted conception. The estimated rates of monozygotic twins was 0.45 and 0.72/100 (95% CI: 0.58-0.91) deliveries after natural and assisted conception, respectively. This difference was statistically significant (p < .05).
  
  
  CONCLUSIONS
  the present population based study suggests that the risk of monozygotic twin is about 60% higher among assisted than after natural conception.
authors.affiliation: Department of Clinical Sciences and Community,University of Milan,Milan,Italy.
authors.forename: Fabio
authors.initials: F
authors.lastname: Parazzini
authors.affiliation: Department of Clinical Sciences and Community,University of Milan,Milan,Italy.
authors.forename: Sonia
authors.initials: S
authors.lastname: Cipriani
authors.affiliation: Department of Clinical Sciences and Community,University of Milan,Milan,Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Bianchi
authors.affiliation: AO Ospedale Maggiore Niguarda,Milano,Italy.
authors.forename: Camilla
authors.initials: C
authors.lastname: Bulfoni
authors.affiliation: Office for Research Promotion,Department of the Hospital Management and Pharmacy,Verona University Hospital,Verona,Italy.
authors.forename: Renata
authors.initials: R
authors.lastname: Bortolus
authors.affiliation: Department Mother and Child,Maggiore Hospital of Milan,University of Milan,Milan,Italy.
authors.forename: Edgardo
authors.initials: E
authors.lastname: Somigliana
mesh_terms: D000328:Adult; D005260:Female; D005307:Fertilization in Vitro / Q000379:methods / Q000706:statistics & numerical data; D006801:Humans; D007558:Italy; D010062:Ovulation Induction / Q000379:methods / Q000706:statistics & numerical data; D011247:Pregnancy; D059285:Pregnancy, Twin; D012098:Reproduction; D027724:Reproductive Techniques, Assisted / Q000706:statistics & numerical data*; D012307:Risk Factors; D014430:Twins, Monozygotic / Q000706:statistics & numerical data*; D055815:Young Adult
keywords: assisted reproduction; monozygotic; twins