Results
A total of 732,810 singleton livebirth and stillbirths were included, among 649,918 women. Of these, 646,926 (88.3%) were by unassisted conception; 68,822 (9.4%) in patients with a history of subfertility; 9024 (1.2%) after non-invasive infertility treatment, and 8038 (1.1%) following invasive infertility treatment (Figure 1). Patients with subfertility, non-invasive infertility and invasive infertility treatment were more likely to be older, nulliparous, obese, and more likely to have diabetes. Chronic hypertension was more common in women with non-invasive and invasive infertility treatment (Table 1).
IPTW achieved a balance in the confounding variables in the weighted sample (Figure S1). There were 45,343 cases of PTB < 37 weeks’ gestation (6.2%), of which 33,117 (4.5%) were spontaneous and 12,226 (1.7%) provider-initiated. Moreover, these figures translate into 73% of all PTB being spontaneous and 27% provider-initiated, further differing depending on the mode of conception (Figure 2). For example, while provider-initiated PTB accounted for 26% of all PTB in women with spontaneous conception (i.e., 10,033/38,444), it contributed to 40% of all PTB in those with invasive infertility treatment (i.e., 350/869).
Relative to births by unassisted conception, the adjusted RR of PTB < 37 weeks was increased in women with subfertility, non-invasive and invasive infertility treatment (Figure 2, top). The adjusted RR of provider-initiated PTB < 37 weeks was higher in women with subfertility (1.23, 95% CI 1.16-1.31), non-invasive infertility treatment (1.48, 1.29-1.69) and invasive infertility treatment (2.35, 2.09-2.64) – each relative to births by unassisted conception (Figure 2). The corresponding adjusted RR for spontaneous PTB < 37 weeks were 1.15 (95% CI 1.10-1.19), 1.19 (95% CI 1.09-1.31) and 1.40 (95% CI 1.27-1.53) (Figure 2).
For the outcome of PTB < 34 weeks’ gestation, the RRs followed a similar pattern as for PTB < 37 weeks (Figure 2). One exception was among women with subfertility, who were not at appreciably higher risk of provider-initiated PTB < 34 weeks’ gestation (adjusted RR 1.08, 95% CI 0.95-1.23).