Twins: Two individuals derived from two FETUSES that were fertilized at or about the same time, developed in the UTERUS simultaneously, and born to the same mother. Twins are either monozygotic (TWINS, MONOZYGOTIC) or dizygotic (TWINS, DIZYGOTIC).Diseases in Twins: Disorders affecting TWINS, one or both, at any age.Twins, Monozygotic: Two off-spring from the same PREGNANCY. They are from a single fertilized OVUM that split into two EMBRYOS. Such twins are usually genetically identical and of the same sex.Twins, ConjoinedTwins, Dizygotic: Two offspring from the same PREGNANCY. They are from two OVA, fertilized at about the same time by two SPERMATOZOA. Such twins are genetically distinct and can be of different sexes.Twin Studies as Topic: Methods of detecting genetic etiology in human traits. The basic premise of twin studies is that monozygotic twins, being formed by the division of a single fertilized ovum, carry identical genes, while dizygotic twins, being formed by the fertilization of two ova by two different spermatozoa, are genetically no more similar than two siblings born after separate pregnancies. (Last, J.M., A Dictionary of Epidemiology, 2d ed)Pregnancy, Twin: The condition of carrying TWINS simultaneously.Pregnancy, Multiple: The condition of carrying two or more FETUSES simultaneously.Fetofetal Transfusion: Passage of blood from one fetus to another via an arteriovenous communication or other shunt, in a monozygotic twin pregnancy. It results in anemia in one twin and polycythemia in the other. (Lee et al., Wintrobe's Clinical Hematology, 9th ed, p737-8)Pregnancy: The status during which female mammals carry their developing young (EMBRYOS or FETUSES) in utero before birth, beginning from FERTILIZATION to BIRTH.Triplets: Three individuals derived from three FETUSES that were fertilized at or about the same time, developed in the UTERUS simultaneously, and born to the same mother.Pregnancy Reduction, Multifetal: Selective abortion of one or more embryos or fetuses in a multiple gestation pregnancy. The usual goal is to improve the outcome for the remaining embryos or fetuses.Environment: The external elements and conditions which surround, influence, and affect the life and development of an organism or population.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.Fetal Death: Death of the developing young in utero. BIRTH of a dead FETUS is STILLBIRTH.Chorion: The outermost extra-embryonic membrane surrounding the developing embryo. In REPTILES and BIRDS, it adheres to the shell and allows exchange of gases between the egg and its environment. In MAMMALS, the chorion evolves into the fetal contribution of the PLACENTA.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.Infant, Newborn: An infant during the first month after birth.Genetics, Behavioral: The experimental study of the relationship between the genotype of an organism and its behavior. The scope includes the effects of genes on simple sensory processes to complex organization of the nervous system.Ultrasonography, Prenatal: The visualization of tissues during pregnancy through recording of the echoes of ultrasonic waves directed into the body. The procedure may be applied with reference to the mother or the fetus and with reference to organs or the detection of maternal or fetal disease.Birth Weight: The mass or quantity of heaviness of an individual at BIRTH. It is expressed by units of pounds or kilograms.Gene-Environment Interaction: The combined effects of genotypes and environmental factors together on phenotypic characteristics.Twinning, Monozygotic: The division of a ZYGOTE into two parts, each of which is capable of further development.Social Environment: The aggregate of social and cultural institutions, forms, patterns, and processes that influence the life of an individual or community.Registries: The systems and processes involved in the establishment, support, management, and operation of registers, e.g., disease registers.Fetoscopy: Endoscopic examination, therapy or surgery of the fetus and amniotic cavity through abdominal or uterine entry.Quadruplets: Four individuals derived from four FETUSES that were fertilized at or about the same time, developed in the UTERUS simultaneously, and born to the same mother.Polyhydramnios: A condition of abnormally high AMNIOTIC FLUID volume, such as greater than 2,000 ml in the LAST TRIMESTER and usually diagnosed by ultrasonographic criteria (AMNIOTIC FLUID INDEX). It is associated with maternal DIABETES MELLITUS; MULTIPLE PREGNANCY; CHROMOSOMAL DISORDERS; and congenital abnormalities.DenmarkDermatoglyphics: The study of the patterns of ridges of the skin of the fingers, palms, toes, and soles.Genetic Predisposition to Disease: A latent susceptibility to disease at the genetic level, which may be activated under certain conditions.Reproductive Techniques, Assisted: Clinical and laboratory techniques used to enhance fertility in humans and animals.Phenotype: The outward appearance of the individual. It is the product of interactions between genes, and between the GENOTYPE and the environment.Twin StudySwedenBirth Order: The sequence in which children are born into the family.