Table 3 Relative Risk (RR)¹ and Ratio of Ratios (RoR)² for the association between; preterm birth, early preterm birth, and at least one twin being classified as low birth weight or small for gestational age and interpregnancy intervals (IPIs) in twin pregnancies

Preterm Birth

IPI^a

1.05 [0.94–1.18]

0.99 [0.90–1.08]

0.97 [0.89–1.05]

1 [referent]

1.04 [0.97–1.12]

1.12 [1.02–1.22]^**

1.25 [1.10–1.41]^***

Post-Birth IPI^a

1.37 [1.22–1.53]^***

1.22 [1.10–1.36]^***

1.08 [0.97–1.20]

1 [referent]

1.01 [0.92–1.11]

0.96 [0.85–1.08]

0.98 [0.80–1.21]

RoR^b

0.77 [0.66–0.91]

0.81 [0.70–0.93]

0.90 [0.79–1.03]

1 [referent]

1.03 [0.92–1.16]

1.17 [1.01–1.35]

1.27 [1.00–1.62]

Early Preterm Birth

IPI

1.41 [1.08–1.83]^*

1.17 [0.94–1.46]

0.99 [0.80–1.23]

1 [referent]

1.16 [0.96–1.39]

1.19 [0.94–1.51]

1.42 [1.01–2.00]^*

Post-Birth IPI

2.03 [1.61–2.55]^***

1.45 [1.16–1.81]^**

0.99 [0.78–1.26]

1 [referent]

0.88 [0.72–1.07]

0.84 [0.66–1.08]

0.73 [0.45–1.19]

RoR

0.69 [0.49–0.99]

0.81 [0.59–1.10]

1.00 [0.73–1.38]

1 [referent]

1.32 [1.01–1.73]

1.41 [1.01–2.00]

1.94 [1.08–3.53]

Small for Gestational Age

IPI

1.20 [0.90–1.59]

1.24 [1.01–1.54]^*

1.14 [0.93–1.40]

1 [referent]

1.14 [0.95–1.36]

1.19 [0.95–1.51]

1.30 [0.90–1.86]

Post-Birth IPI

1.50 [1.17–1.92]^***

1.13 [0.90–1.42]

0.85 [0.67–1.08]

1 [referent]

0.86 [0.71–1.05]

0.90 [0.71–1.14]

0.93 [0.61–1.41]

RoR

0.80 [0.55–1.16]

1.10 [0.80–1.50]

1.34 [0.98–1.83]

1 [referent]

1.32 [1.01–1.72]

1.33 [0.95–1.85]

1.39 [0.80–2.42]

Low Birth Weight

IPI

1.16 [1.06–1.28]^**

1.09 [1.01–1.19]^*

1.01 [0.93–1.09]

1 [referent]

1.06 [0.99–1.14]

1.17 [1.08–1.28]^***

1.20 [1.05–1.36]^**

Post-Birth IPI

1.30 [1.19–1.42]^***

1.17 [1.07–1.27]^***

1.03 [0.94–1.12]

1 [referent]

1.01 [0.93–1.09]

0.99 [0.90–1.09]

0.87 [0.72–1.05]

RoR

0.90 [0.78–1.02]

0.93 [0.83–1.05]

0.98 [0.87–1.11]

1 [referent]

1.05 [0.95–1.17]

1.19 [1.05–1.35]

1.38 [1.10–1.73]