Inclusion Criteria:  mother's age 18 - 45 years; delivery at term (≥ 37 + 0/7 weeks of pregnancy) or close to the delivery date (from 35 + 0/7 to 36 + 6/7 weeks of pregnancy), both by vaginal delivery and by caesarean section; single pregnancy; good condition of the child after birth, rated > 7 points on the Apgar score after the 1 st minute of life; feeding the baby only naturally (with breast or expressed breast milk) or mainly naturally; mother's informed and voluntary consent to participate in the study; mother's informed and voluntary consent to the participation of her child in the study.  Exclusion Criteria:  mother's lack of consent to participate in the study; mother's lack of consent to the participation of her child in the study; mother's age <18 years and > 45 years; preterm labor <35 + 0/7 weeks of pregnancy; multiple pregnancy; the child's condition at birth is moderate or severe, rated at ≤ 7 points on the Apgar score after the 1st minute of life; feeding a child exclusively or mainly with an infant formula; severe birth defects of a newborn, any clinical condition of the mother and / or the newborn that may affect the nutritional status of the newborn (IUGR, lack of medical care during pregnancy, addiction of the mother to alcohol or other psychoactive substances, nicotinism in pregnancy, uncontrolled asthma in the mother, metabolic diseases in the mother or newborn).