Premature Birth: CHILDBIRTH before 37 weeks of PREGNANCY (259 days from the first day of the mother's last menstrual period, or 245 days after FERTILIZATION).Infant, Premature: A human infant born before 37 weeks of GESTATION.Infant, Newborn: An infant during the first month after birth.Pregnancy: The status during which female mammals carry their developing young (EMBRYOS or FETUSES) in utero before birth, beginning from FERTILIZATION to BIRTH.Obstetric Labor, Premature: Onset of OBSTETRIC LABOR before term (TERM BIRTH) but usually after the FETUS has become viable. In humans, it occurs sometime during the 29th through 38th week of PREGNANCY. TOCOLYSIS inhibits premature labor and can prevent the BIRTH of premature infants (INFANT, PREMATURE).Birth Weight: The mass or quantity of heaviness of an individual at BIRTH. It is expressed by units of pounds or kilograms.Infant, Low Birth Weight: An infant having a birth weight of 2500 gm. (5.5 lb.) or less but INFANT, VERY LOW BIRTH WEIGHT is available for infants having a birth weight of 1500 grams (3.3 lb.) or less.Gestational Age: The age of the conceptus, beginning from the time of FERTILIZATION. In clinical obstetrics, the gestational age is often estimated as the time from the last day of the last MENSTRUATION which is about 2 weeks before OVULATION and fertilization.Pregnancy Outcome: Results of conception and ensuing pregnancy, including LIVE BIRTH; STILLBIRTH; SPONTANEOUS ABORTION; INDUCED ABORTION. The outcome may follow natural or artificial insemination or any of the various ASSISTED REPRODUCTIVE TECHNIQUES, such as EMBRYO TRANSFER or FERTILIZATION IN VITRO.Congenital Abnormalities: Malformations of organs or body parts during development in utero.Abortion, Veterinary: Premature expulsion of the FETUS in animals.Infant, Premature, DiseasesPregnancy Complications: Conditions or pathological processes associated with pregnancy. They can occur during or after pregnancy, and range from minor discomforts to serious diseases that require medical interventions. They include diseases in pregnant females, and pregnancies in females with diseases.Bronchopulmonary Dysplasia: A chronic lung disease developed after OXYGEN INHALATION THERAPY or mechanical ventilation (VENTILATION, MECHANICAL) usually occurring in certain premature infants (INFANT, PREMATURE) or newborn infants with respiratory distress syndrome (RESPIRATORY DISTRESS SYNDROME, NEWBORN). Histologically, it is characterized by the unusual abnormalities of the bronchioles, such as METAPLASIA, decrease in alveolar number, and formation of CYSTS.Fetal Death: Death of the developing young in utero. BIRTH of a dead FETUS is STILLBIRTH.Abortion, Spontaneous: Expulsion of the product of FERTILIZATION before completing the term of GESTATION and without deliberate interference.Infant Mortality: Postnatal deaths from BIRTH to 365 days after birth in a given population. Postneonatal mortality represents deaths between 28 days and 365 days after birth (as defined by National Center for Health Statistics). Neonatal mortality represents deaths from birth to 27 days after birth.Infant, Small for Gestational Age: An infant having a birth weight lower than expected for its gestational age.Pregnancy Complications, Infectious: The co-occurrence of pregnancy and an INFECTION. The infection may precede or follow FERTILIZATION.Respiratory Distress Syndrome, Newborn: A condition of the newborn marked by DYSPNEA with CYANOSIS, heralded by such prodromal signs as dilatation of the alae nasi, expiratory grunt, and retraction of the suprasternal notch or costal margins, mostly frequently occurring in premature infants, children of diabetic mothers, and infants delivered by cesarean section, and sometimes with no apparent predisposing cause.Animals, Newborn: Refers to animals in the period of time just after birth.Birth Rate: The number of births in a given population per year or other unit of time.Risk Factors: An aspect of personal behavior or lifestyle, environmental exposure, or inborn or inherited characteristic, which, on the basis of epidemiologic evidence, is known to be associated with a health-related condition considered important to prevent.Retrospective Studies: Studies used to test etiologic hypotheses in which inferences about an exposure to putative causal factors are derived from data relating to characteristics of persons under study or to events or experiences in their past. The essential feature is that some of the persons under study have the disease or outcome of interest and their characteristics are compared with those of unaffected persons.Birth Order: The sequence in which children are born into the family.Birth Certificates: Official certifications by a physician recording the individual's birth date, place of birth, parentage and other required identifying data which are filed with the local registrar of vital statistics.Birth Intervals: The lengths of intervals between births to women in the population.Birth Injuries: Mechanical or anoxic trauma incurred by the infant during labor or delivery.Multiple Birth Offspring: The offspring in multiple pregnancies (PREGNANCY, MULTIPLE): TWINS; TRIPLETS; QUADRUPLETS; QUINTUPLETS; etc.Maternal Age: The age of the mother in PREGNANCY.Delivery, Obstetric: Delivery of the FETUS and PLACENTA under the care of an obstetrician or a health worker. Obstetric deliveries may involve physical, psychological, medical, or surgical interventions.Vaginal Birth after Cesarean: Delivery of an infant through the vagina in a female who has had a prior cesarean section.Parity: The number of offspring a female has borne. It is contrasted with GRAVIDITY, which refers to the number of pregnancies, regardless of outcome.Cohort Studies: Studies in which subsets of a defined population are identified. These groups may or may not be exposed to factors hypothesized to influence the probability of the occurrence of a particular disease or other outcome. Cohorts are defined populations which, as a whole, are followed in an attempt to determine distinguishing subgroup characteristics.