Nulliparity is associated with lower birthweight, but few studies have examined how within‐mother changes in risk factors impact this association.

We used longitudinal electronic medical record data from a hospital‐based cohort of consecutive singleton live births from 2002–2010 in Utah. To reduce bias from unobserved pregnancies, primary analyses were limited to 9484 women who entered nulliparous from 2002–2004, with 23 380 pregnancies up to parity 3. Unrestricted secondary analyses used 101 225 pregnancies from 45 212 women with pregnancies up to parity 7. We calculated gestational age and sex‐specific birthweight z‐scores with nulliparas as the reference. Using linear mixed models, we estimated birthweight z‐score by parity adjusting for pregnancy‐specific sociodemographics, smoking, alcohol, prepregnancy body mass index, gestational weight gain, and medical conditions.

Compared with nulliparas’, infants of primiparas were larger by 0.20 unadjusted z‐score units [95% confidence interval (CI) 0.18, 0.22]; the adjusted increase was similar at 0.18 z‐score units [95% CI 0.15, 0.20]. Birthweight continued to increase up to parity 3, but with a smaller difference (parity 3 vs. 0 β = 0.27 [95% CI 0.20, 0.34]). In the unrestricted secondary sample, there was significant departure in linearity from parity 1 to 7 (P < 0.001); birthweight increased only up to parity 4 (parity 4 vs. 0 β = 0.34 [95% CI 0.31, 0.37]).

The association between parity and birthweight was non‐linear with the greatest increase observed between first‐ and second‐born infants of the same mother. Adjustment for changes in weight or chronic diseases did not change the relationship between parity and birthweight.