The objective was to evaluate maternal Mediterranean diet (MD) pattern adherence during pregnancy and its association with small for gestational age (SGA) and preterm birth. A secondary objective of the current study was to describe the sociodemographic, lifestyle and obstetric profile of the mothers studied as well as the most relevant paternal and newborn characteristics. The current study is a two-phase retrospective population-based study of maternal dietary habits during pregnancy and their effect on newborn size and prematurity. The descriptive first phase examined maternal dietary habits during pregnancy along with the maternal sociodemographic, lifestyle and obstetric profile in a cross-sectional period study. In the second phase, newborn outcomes were evaluated in a nested case-control study. Adherence to MD during pregnancy was measured with the Spanish version of Kidmed index. Obstetrics ward of the La Fe Hospital in Valencia. All mother-child pairs admitted after delivery during a 12-month period starting from January 2018 were assessed for eligibility. A total of 1118 provided complete outcome data after signing informed consent. 14·5 % met the criteria of poor adherence (PA); 34·8 %, medium adherence (MA); and 50·7 %, optimal adherence (OA). Medium adherence to MD was associated in the adjusted scenarios with a higher risk of giving birth to a preterm newborn. No association was found between MD adherence and SGA. Early intervention programmes geared towards pregnant women, where women were aided in reaching OA to MD, might reduce the risk of preterm newborn.