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Summary The effect is described of maternal factors on the proportion of fetal chromosomal anomalies in a series of 2620 spontaneous abortions, of which 992 specimens were karyotyped. Maternal age was the most important factor associated with a rise in the proportion abnormal, followed by Social Class I or II and the use of oral contraception before conception. The problem of extrapolating from the proportion abnormal to absolute incidence of anomalies is discussed. In the case of increasing maternal age, the evidence suggests that the rise in incidence of spontaneous abortions with age is accounted for by an increased incidence of chromosomally abnormal fetuses. In the case of high social class and a history of oral contraception, the evidence on incidence is scanty and the rise in the proportion abnormal may either reflect a decrease in the abortion rate of chromosomally normal fetuses, or a small increase in the incidence of lethal chromosomal anomalies.

Summary Information on the previous reproductive history of 3467 mothers presenting with a spontaneous abortion is compared with that of a control series presenting with a livebirth. In 1384 abortions the chromosome constitution of the fetal products was determined. The mothers who had delivered a chromosomally normal abortion more often had a history of previous repeated abortions, while those of chromosomally abnormal abortions tended to have fewer repeated abortions but an excess of previous viable children with Down's syndrome. Data are also presented on 23 mothers with two spontaneous abortions of known karyotype; these, together with similar data obtained elsewhere, made it possible to estimate an overall rate of recurrence of chromosome defects in conceptions of the same parents, and the probability that affected fetuses will survive to a viable stage in pregnancy. The clinical and genetic implications of the findings are briefly discussed.

STUDY QUESTION What is the status of fertility treatment and birth outcomes documented over the first 6 years of the Canadian Assisted Reproductive Technologies Register (CARTR) Plus registry? SUMMARY ANSWER The CARTR Plus registry is a robust database containing comprehensive Canadian fertility treatment data to assist with providing evidence-based rationale for clinical practice change. WHAT IS KNOWN ALREADY The rate of infertility is increasing globally and having data on fertility treatment cycles and outcomes at a population level is important for accurately documenting and effecting changes in clinical practice. STUDY DESIGN, SIZE, DURATION This is a descriptive manuscript of 183 739 fertility treatment cycles from 36 Canadian clinics over 6 years from the CARTR Plus registry. PARTICIPANTS/MATERIALS, SETTING, METHODS Canadian ART treatment cycles from 2013 through 2018 were included. This manuscript described trends in type of fertility treatment cycles, pregnancy rates, multiple pregnancy rates, primary transfer rates and birth outcomes. MAIN RESULTS AND THE ROLE OF CHANCE Over the 6 years of the CARTR Plus registry, the number of treatment cycles performed ranged from less than 200 to greater than 1000 per clinic. Patient age and the underlying cause of infertility were two of the most variable characteristics across clinics. Similar clinical pregnancy rates were found among IVF and frozen embryo transfer (FET) cycles with own oocytes (38.9 and 39.7% per embryo transfer cycle, respectively). Fertility treatment cycles that used donor oocytes had a higher clinical pregnancy rate among IVF cycles compared with FET cycles (54.9 and 39.8% per embryo transfer cycle, respectively). The multiple pregnancy rate was 7.4% per ongoing clinical pregnancy in 2018, which reflected a decreasing trend across the study period. Between 2013 and 2017, there were 31 811 pregnancies that had live births from all ART treatment cycles, which corresponded to a live birth rate of 21.4% per cycle start and 89.1% of these pregnancies were singleton live births. The low multiple pregnancy rate and high singleton birth rate are associated with the increase in single embryo transfers. LIMITATIONS, REASONS FOR CAUTION There is potential for misclassification of data, which is present in all administrative health databases. WIDER IMPLICATIONS OF THE FINDINGS The CARTR Plus registry is a robust resource for ART data in Canada. It provides easily accessible aggregated data for Canadian fertility clinics, and it contains data that are internationally comparable. STUDY FUNDING/COMPETING INTEREST(S) There was no funding provided for this study. The authors have no competing interests to declare.

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