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 (http://creativecommons.org/licenses/by-nc-nd/4.0/). investigate the interaction between maternal booking BMI, gestational weight gain (GWG) and neonatal birthweight centiles. Methods: 16.5 year-observational cohort study (2001e2017). 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) ¼ À1.2 ppBMI (Kg/m 2) þ 42 AE 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 AE 2 kg (instead of 12.5e18). An obese 32 kg/m 2 should gain 3.6 kg (instead of 5e9). Very obese 40 kg/m 2 should lose 6 kg.