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Abstract
Background and objectives. Birth size curves are needed for clinical and epidemiological purposes. We constructed birth weight (BW), length (BL), and head circumference (BHC) references, assessed effects of twinness and parity, and defined cut-off points for small, appropriate, and large for gestational age.
Materials and methods. Birth register data of all 753,036 infants born in 1996–2008 in Finland were cleaned to create references reflecting optimal intrauterine growth. The final data included 533,666 singletons and 15,033 twins (median gestation weeks (gws) 40.0 and 37.1, respectively, 41.6% primiparous). Sex-specific BW, BL, and BHC references were constructed from 23 to 43 gws separately for singletons and twins born to primiparous or multiparous mothers. GAMLSS method was used for modelling.
Results. In singletons from 36 gws onwards, increased BW and BL were observed in comparison to previous reference from 1979–1983. Twins diverged from singletons from 30 gws onwards. At 37.0 gws, mean BW was 400 g lower and mean BL 1.2 cm shorter than in singletons. From 30 gws onwards, birth size was larger in infants of multiparous than primiparous mothers.
Conclusions. Population-based birth size references are available for the evaluation of birth size. Accounting for plurality and parity improves the accuracy of birth size evaluation.
Declaration of interest: This work was supported by the Kuopio University Hospital, the Paediatric Research Foundation, the National Graduate School of Clinical Investigation, and the Finnish Funding Agency for Technology and Innovation (Tekes). None of the study sponsors has participated in designing the study, analysing the data, writing the report, or submitting it for publication. None of the authors has financial relationships with the funders. The authors report no conflicts of interest.