Maternal age categories (in 5-year intervals: 15–19, 20–24, 25–29 years, etc.), as well as pre-pregnancy body mass index (BMI), also categorized in 5 kg/m² increments (15–19, 20–24 kg/m², etc.), both show highly consistent linear associations with major maternal and fetal morbidities, suggesting the existence of underlying biological governing principles.

1) Rate of cesarean sections

a) Maternal age and primiparity: The rates of cesarean section, vaginal delivery, operative vaginal delivery (vacuum, forceps, etc.), as well as the incidences of term breech presentation and placenta praevia, follow reproducible linear equations according to maternal age.

b) Pre-pregnancy BMI and primiparity: Similarly, in relation to pre-pregnancy BMI and primiparity, the rate of cesarean section also follows a robust linear equation.

2) Defining optimal gestational weight gain (GWG) for the index pregnancy in primiparous and multiparous women

In 2018, through a serendipitous analysis of graphical data, we observed that for each pre-pregnancy BMI category (in 5 kg/m² increments), there was a consistent crossover point corresponding to 10% of the neonatal population, with both 10% small-for-gestational-age (SGA) and 10% large-for-gestational-age (LGA) infants. Across BMI categories, from underweight to super-obese (≥ 40 kg/m²), this pattern demonstrated a regular and remarkably linear shift from high GWG (more than 20 kg) in lower BMI groups (underweight women, < 18.5 kg/m²) to very low or even negative GWG in severe obesity.

For singleton pregnancies, this relationship can be expressed as:

    optimal GWG (kg) = -1.2 × pre-pregnancy BMI (kg/m²) + 42 ± 2 kg  

Application of this equation may reduce the rate of cesarean sections, the incidence of large-for-gestational-age newborns (LGA), macrosomic infants (≥ 4 kg), and late-onset preeclampsia (≥ 34 weeks’ gestation; representing 80–90% of preeclampsia cases in a population) in severely obese women (> 35 kg/m²). In underweight women (< 18.5 kg/m²), it may help reduce the rate of small-for-gestational-age newborns (SGA).

Conclusion: These highly reproducible linear associations may reflect law-like biological relationships in human reproduction and suggest fundamental physiological principles that warrant further investigation. They also have immediate practical implications:

First, they enable prediction of major maternal and fetal morbidities from the very beginning of pregnancy. Second, calculation of optimal gestational weight gain may help reduce these important complications. Third, because of these reproducible linear patterns, maternal age and pre-pregnancy BMI (whether divided into 5-unit increments or treated as continuous variables)should in the future be incorporated into nearly all logistic models used in epidemiological perinatal studies. These reproducible linear associations may represent mathematical laws of human reproduction, pointing toward fundamental biological principles that deserve further investigation.

Pierre-Yves Robillard is a neonatologist, epidemiologist, and specialist in tropical diseases and perinatal epidemiology. He has 46 years of work in tropical countries (French overseas departments and territories): 16 years in Guadeloupe (1979–1995, French West-Indies, Caribbean), 3 years in Tahiti (1995–1998, French Polynesia, Pacific), and 26 years (since 1999) in Reunion, a French overseas department in the Indian Ocean. He has spent his entire career in level 3 NICUs and university hospitals. He served for twelve years (2002–2014) at the head of the neonatal department and NICU of the University Hospital South Reunion (Saint-Pierre). He is currently working at Centre Hospitalier Universitaire Sud-Réunion, Saint-Pierre. He completed one year of post-doctoral fellowship in perinatal epidemiology at MUSC (Medical University of South Carolina, Charleston, USA, 1991–1992) and attended the International Course of Epidemiology at CDC Atlanta (1992).