Relationship between pre-pregnancy maternal BMI and optimal weight gain in singleton pregnancies

BACKGROUND: There is a peculiar phenomenon: two separate individuals (mother and foetus) have a mutually interactive dependency concerning their respective weight. Very thin mothers have a higher risk of small for gestational age (SGA) infants, and rarely give birth to a large for gestational age (LGA) infant. While morbidly obese women often give birth to LGA infants, and rarely to SGA. Normal birthweight (AGA) infants (>10(th) and <90(th) centile of a neonatal population) typically have the lowest perinatal and long-term morbidity. The aim of the current study is (1) to determine the maternal body mass index (BMI) range associated with a balanced risk (10% SGA, 10% LGA), and (2) to investigate the interaction between maternal booking BMI, gestational weight gain (GWG) and neonatal birthweight centiles. METHODS: 16.5 year-observational cohort study (2001–2017). The study population consisted of all consecutive singleton term (37 weeks onward) live births delivered at University's maternity in Reunion island, French Overseas Department. FINDINGS: Of the 59,717 singleton term live births, we could define the booking BMI and the GWG in 52,092 parturients (87.2%). We had 2 major findings (1) Only women with a normal BMI achieve an equilibrium in the SGA/LGA risk (both 10%). We propose to call this crossing point the Maternal Fetal Corpulence Symbiosis (MFCS). (2) This MFCS shifts with increasing GWG. We tested the MFCS by 5 kg/m(2) incremental BMI categories. The result is a linear law: opGWG (kg) = −1.2 ppBMI (Kg/m²) + 42 ± 2 kg INTERPRETATION: IOM-2009 recommendations are adequate for normal and over-weighted women but not for thin and obese women: a thin woman (17 kg/m(2)) should gain 21.6 ± 2 kg (instead of 12.5–18). An obese 32 kg/m(2) should gain 3.6 kg (instead of 5–9). Very obese 40 kg/m(2) should lose 6 kg.