All naturally conceived liveborn singletons delivered at St. Mary’s Community Hospital in Montreal, Canada, and liveborn singletons resulting from IVF treatment at the university affiliated fertility center OVO clinic in Montreal, Canada, between 2010 and 2014 were selected. A total number of 20,713 deliveries were evaluated, among them 16,616 cases met the inclusion criteria and were incorporated in the analysis.
The inclusion criteria comprised of the following conditions: maternal age of 41 years and below, gestational age at the time of delivery of 20 weeks and above, birth weight of 600 grams and above, and a BMI of 35 kg/m2 and below. Only patients who received a sET were included, whereas women who received more than one embryo per transfer were excluded. Moreover, multiple births were excluded from the analysis.
The births were distributed according to the mode of conception. Thus, patients were categorized into four groups: (group 1) babies conceived naturally without any medical assistance in normal fertile couples (NAT, n=15,770); (group 2) babies conceived through mnIVF (n=235); (group 3) babies conceived sIVF (n=389); and (group 4) babies conceived through IVF but only following FET (n=222). In the previously mentioned ART protocols, embryos were transferred on day 2, 3, or 5 post-fertilization.
After obtaining the approval of the scientific-ethical committee at OVO clinic as a quality control, data were retrospectively retrieved between 2010 and 2014. A chart review of the electronic and paper records of all included cases was performed. Demographic information including maternal age, infertility type (primary, secondary), years of infertility, BMI, ovarian reserve, AMH levels, smoking status; treatment variables including ART protocol (mnIVF, sIVF, FET), insemination type (conventional IVF, ICSI), type of embryo, the day of embryo transfer; and perinatal outcomes including gestational age, birth weight, sex of the baby, and mode of delivery (cesarean, vaginal) were collected.
The modified natural IVF protocol consisted of monitoring follicular development with trans-vaginal ultrasound. When the leading follicle reached a diameter of 15 mm with an endometrial thickness ≥6mm, human menopausal gonadotropin (hMG) injections (Menopur or Repronex, Ferring Pharmaceuticals, Canada) at a dose of 150 IU daily along with indomethacin 50 mg orally three times daily and GnRH antagonist Orgalotran 250 mcg injection daily (Merck pharmaceutical, Canada) were started until the day of trigger of final oocyte maturation. Follow-up ultrasound was performed accordingly. When the leading follicle reached a diameter between 17 and 20 mm, estradiol and progesterone dosages were performed for possible cancelation and hCG triggering using 5000 IU followed by ovum pick up 34 h later on was conducted.
Stimulated IVF cycles were performed using standard GnRH antagonist, long agonist, or micro-dose flare protocols. Follicular development and estradiol levels were monitored during the stimulation and on the day of trigger. When 3 follicles measuring ≥ 18 mm were obtained, hCG triggering was administered. Oocyte retrieval was performed 36 h after triggering of final oocyte maturation. ICSI was performed for male factor, unexplained infertility, prior fertilization failure with conventional insemination, preimplantation genetic testing (PGT), poor-quality oocytes, low oocyte yield, and advanced maternal age. Fresh embryo transfer was performed if the risk of ovarian hyper-stimulation syndrome (OHSS) was deemed low.
For the FET cycles, endometrial preparation was carried out using different protocols based on the clinical background of patients, their prior response if available, and the discretion of the physician. The protocols included either FET in a natural cycle, hormone replacement conventional substituted, or stimulated.
The gestational age at the time of delivery and birth weight were considered the main outcomes, whereas the sex of the baby and the mode of delivery were considered as secondary outcomes. Obstetrical complications were not covered by this study.
Statistical analyses were performed using the SAS/STAT® software (SAS University Edition, version 9.4M5; SAS Institute Inc. Cary NC, USA). Data were tested for normality by Kolmogorov-Smirnov test and transformed to natural logarithms or ranks as appropriate. NAT, mnIVF, sIVF, and FET were compared for the demographic information, the treatment variables and the perinatal outcomes previously mentioned.
In order to determine the relationship between each one of the a foure mentioned groups, as explanatory variables, and perinatal outcomes as dependent variables, a univariate analysis was performed. Chi-square test was performed for categorical variables, whereas Student’s t test was performed for continuous variables. A p value <0.05 was considered as statistically significant in all tests. Results were presented as mean ± standard deviation (SD) or as percentages. Moreover, a head-to-head comparison was performed between each two groups alone for both primary outcomes. Results for the multiple comparison were presented as mean difference with 95% confidence interval and a p value.
To control for potentially confounding variables, conception type, years of infertility, type of insemination, smoking status, AMH level, category of BMI, type of infertility, ovarian reserve, day of transfer, BMI, and maternal age were included in a backward regression analysis in the ART groups for gestational age. Similarly, the same variables in addition to the gestational age were included in a backward regression analysis in the ART groups for birth weight. The selection method in this analysis consists of entering the independent variables into the equation first and each factor is then deleted one at a time if they do not contribute to the regression equation. The strength for the effect size in the backward regression analysis was evaluated using the R-squared value.