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).