We included 75,939 pregnant women in the Norwegian Mother and Child Cohort Study. Data were obtained by self-administered questionnaires.
By pregnancy week 30, 15 % of the women had developed PGS. Among first-time mothers, 11 % of the women reported PGS, compared with 18 % of the women with 1 previous delivery and 21 % of women with 2 previous deliveries. The odds ratios for PGS of having had 1 or 2 previous deliveries were 1.9 (95 % confidence interval [CI], 1.9–2.0) and 2.4 (95 % CI, 2.3–2.6), respectively, after adjustment for other study factors. For PGS with severe pain, the corresponding odds ratios were 2.6 (95 % CI, 2.3–2.9) and 3.8 (95 % CI, 3.3–4.3).
The risk of the development of PGS increased with number of previous deliveries, which suggests that parity-related factors play a causal role.