The aim of this study was to evaluate the relationship between advanced maternal age and prematurity (both spontaneous and iatrogenic) after controlling for multiple confounders . During the past ten years, a gradual increase in maternal age has been observed worldwide.

Maternal age at pregnancy is increasing worldwide as well as preterm birth. However, the association between prematurity and advanced maternal age remains controversial.

A study on more than 80,000 women showed that 36% of the increase in prematurity , between 1990 and 1996 in Canada, was attributable to the change towards increasing maternal age.

Several studies have tried to study the specific influence of advanced maternal age after adjustment for hypertensive disorders of pregnancy, maternal medical history or assisted reproductive technologies , but the evidence is still conflicting.

Retrospective cohort study using data from the QUARISM study , a large Canadian randomized controlled trial, which collected data from 184,000 births in 32 hospitals. Inclusion criteria were maternal age over 20 years. Exclusion criteria were multiple pregnancy, fetal malformation and intra-uterine fetal death.

Five maternal age categories were defined and compared for maternal characteristics, gestational and obstetric, and risk factors for prematurity. Risk factors for preterm birth <37 weeks, either spontaneous or iatrogenic, were evaluated for different age groups using multivariate logistic regression .

Of the 165,282 births were included in the study. Chronic hypertension, assisted reproductive techniques, pre-gestational diabetes, invasive procedure in pregnancy, gestational diabetes and placenta praevia were linearly associated with increasing maternal age and hypertensive disorders of pregnancy followed by "U" shaped distribution according to maternal age.

Crude rates of preterm birth before 37 weeks followed by "U" shaped curve with a nadir at 5.7% for the group of 30-34 years. In multivariate analysis, the adjusted odds ratio (aOR) of prematurity stratified by age group followed by "U" shaped distribution with an OR of 1.08 (95% CI; 1.01-1.15) for 20-24 years, and 1.20 (95% CI; 1.06-1.36) for 40 years and older.

Confounders found to have the greatest impact were pranavia placenta , hypertension disorders, and maternal medical history. Even after adjustment for confounders, advanced maternal age (40 years and over) was associated with preterm birth. A maternal age of 30-34 years was associated with the lowest risk of prematurity .