Cervical Length Measurement: A parameter usually used in PRENATAL ULTRASONOGRAPHY to measure the length of the uterine neck (CERVIX UTERI). Cervical length or its shortening is used to identify and prevent early cervical opening and PRETERM BIRTH.Cervix Uteri: The neck portion of the UTERUS between the lower isthmus and the VAGINA forming the cervical canal.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).Cervical Ripening: A change in the CERVIX UTERI with respect to its readiness to relax. The cervix normally becomes softer, more flexible, more distensible, and shorter in the final weeks of PREGNANCY. These cervical changes can also be chemically induced (LABOR, INDUCED).Pregnancy Trimester, Second: The middle third of a human PREGNANCY, from the beginning of the 15th through the 28th completed week (99 to 196 days) of gestation.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).Pregnancy Trimester, Third: The last third of a human PREGNANCY, from the beginning of the 29th through the 42nd completed week (197 to 294 days) of gestation.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.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: The status during which female mammals carry their developing young (EMBRYOS or FETUSES) in utero before birth, beginning from FERTILIZATION to BIRTH.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).Uterine Cervical Incompetence: Incompetent UTERINE CERVIX is usually diagnosed in the second trimester of PREGNANCY. It is characterized by passive painless cervical dilation in the absence of UTERINE CONTRACTION; BLEEDING; INFECTION; and sometimes with the amniotic sac (AMNIOTIC MEMBRANE) bulging through the partially dilated cervix. Left untreated, this condition may lead to premature pregnancy loss, such as HABITUAL ABORTION.Cerclage, Cervical: The surgical closure of the incompetent cervix uteri with suture material.Pregnancy, Prolonged: A term used to describe pregnancies that exceed the upper limit of a normal gestational period. In humans, a prolonged pregnancy is defined as one that extends beyond 42 weeks (294 days) after the first day of the last menstrual period (MENSTRUATION), or birth with gestational age of 41 weeks or more.Axial Length, Eye: The distance between the anterior and posterior poles of the eye, measured either by ULTRASONOGRAPHY or by partial coherence interferometry.Labor, Induced: Artificially induced UTERINE CONTRACTION. Generally, LABOR, OBSTETRIC is induced with the intent to cause delivery of the fetus and termination of pregnancy.Labor Onset: The beginning of true OBSTETRIC LABOR which is characterized by the cyclic uterine contractions of increasing frequency, duration, and strength causing CERVICAL DILATATION to begin (LABOR STAGE, FIRST ).Nasal Bone: Either one of the two small elongated rectangular bones that together form the bridge of the nose.Interferometry: Measurement of distances or movements by means of the phenomena caused by the interference of two rays of light (optical interferometry) or of sound (acoustic interferometry).Gravidity: The number of pregnancies, complete or incomplete, experienced by a female. It is different from PARITY, which is the number of offspring borne. (From Stedman, 26th ed)Pregnancy, High-Risk: Pregnancy in which the mother and/or FETUS are at greater than normal risk of MORBIDITY or MORTALITY. Causes include inadequate PRENATAL CARE, previous obstetrical history (ABORTION, SPONTANEOUS), pre-existing maternal disease, pregnancy-induced disease (GESTATIONAL HYPERTENSION), and MULTIPLE PREGNANCY, as well as advanced maternal age above 35.Biometry: The use of statistical and mathematical methods to analyze biological observations and phenomena.Crown-Rump Length: In utero measurement corresponding to the sitting height (crown to rump) of the fetus. Length is considered a more accurate criterion of the age of the fetus than is the weight. The average crown-rump length of the fetus at term is 36 cm. (From Williams Obstetrics, 18th ed, p91)Tocolysis: Any drug treatment modality designed to inhibit UTERINE CONTRACTION. It is used in pregnant women to arrest PREMATURE LABOR.Reproducibility of Results: The statistical reproducibility of measurements (often in a clinical context), including the testing of instrumentation or techniques to obtain reproducible results. The concept includes reproducibility of physiological measurements, which may be used to develop rules to assess probability or prognosis, or response to a stimulus; reproducibility of occurrence of a condition; and reproducibility of experimental results.Abortion, Threatened: UTERINE BLEEDING from a GESTATION of less than 20 weeks without any CERVICAL DILATATION. It is characterized by vaginal bleeding, lower back discomfort, or midline pelvic cramping and a risk factor for MISCARRIAGE.Pregnancy Trimester, First: The beginning third of a human PREGNANCY, from the first day of the last normal menstrual period (MENSTRUATION) through the completion of 14 weeks (98 days) of gestation.Telomere Homeostasis: Maintenance of TELOMERE length. During DNA REPLICATION, chromosome ends loose some of their telomere sequence (TELOMERE SHORTENING.) Various cellular mechanism are involved in repairing, extending, and recapping the telomere ends.Observer Variation: The failure by the observer to measure or identify a phenomenon accurately, which results in an error. Sources for this may be due to the observer's missing an abnormality, or to faulty technique resulting in incorrect test measurement, or to misinterpretation of the data. Two varieties are inter-observer variation (the amount observers vary from one another when reporting on the same material) and intra-observer variation (the amount one observer varies between observations when reporting more than once on the same material).Telomere: A terminal section of a chromosome which has a specialized structure and which is involved in chromosomal replication and stability. Its length is believed to be a few hundred base pairs.Prospective Studies: Observation of a population for a sufficient number of persons over a sufficient number of years to generate incidence or mortality rates subsequent to the selection of the study group.Parity: The number of offspring a female has borne. It is contrasted with GRAVIDITY, which refers to the number of pregnancies, regardless of outcome.Pregnancy, Multiple: The condition of carrying two or more FETUSES simultaneously.Cesarean Section: Extraction of the FETUS by means of abdominal HYSTEROTOMY.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.Predictive Value of Tests: In screening and diagnostic tests, the probability that a person with a positive test is a true positive (i.e., has the disease), is referred to as the predictive value of a positive test; whereas, the predictive value of a negative test is the probability that the person with a negative test does not have the disease. Predictive value is related to the sensitivity and specificity of the test.Telomere Shortening: The loss of some TELOMERE sequence during DNA REPLICATION of the first several base pairs of a linear DNA molecule; or from DNA DAMAGE. Cells have various mechanisms to restore length (TELOMERE HOMEOSTASIS.) Telomere shortening is involved in the progression of CELL AGING.Body Weights and Measures: Measurements of the height, weight, length, area, etc., of the human and animal body or its parts.Sensitivity and Specificity: Binary classification measures to assess test results. Sensitivity or recall rate is the proportion of true positives. Specificity is the probability of correctly determining the absence of a condition. (From Last, Dictionary of Epidemiology, 2d ed)Fetal Membranes, Premature Rupture: Spontaneous tearing of the membranes surrounding the FETUS any time before the onset of OBSTETRIC LABOR. Preterm PROM is membrane rupture before 37 weeks of GESTATION.Hydroxyprogesterones: Metabolites or derivatives of PROGESTERONE with hydroxyl group substitution at various sites.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.ROC Curve: A graphic means for assessing the ability of a screening test to discriminate between healthy and diseased persons; may also be used in other studies, e.g., distinguishing stimuli responses as to a faint stimuli or nonstimuli.Reference Values: The range or frequency distribution of a measurement in a population (of organisms, organs or things) that has not been selected for the presence of disease or abnormality.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.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.Anterior Chamber: The space in the eye, filled with aqueous humor, bounded anteriorly by the cornea and a small portion of the sclera and posteriorly by a small portion of the ciliary body, the iris, and that part of the crystalline lens which presents through the pupil. (Cline et al., Dictionary of Visual Science, 4th ed, p109)Anthropometry: The technique that deals with the measurement of the size, weight, and proportions of the human or other primate body.Labor Stage, First: Period from the onset of true OBSTETRIC LABOR to the complete dilatation of the CERVIX UTERI.Uterine Contraction: Contraction of the UTERINE MUSCLE.Pessaries: Devices worn in the vagina to provide support to displaced uterus or rectum. Pessaries are used in conditions such as UTERINE PROLAPSE; CYSTOCELE; or RECTOCELE.Infant, Newborn: An infant during the first month after birth.Vagina: The genital canal in the female, extending from the UTERUS to the VULVA. (Stedman, 25th ed)Administration, Intravaginal: The insertion of drugs into the vagina to treat local infections, neoplasms, or to induce labor. The dosage forms may include medicated pessaries, irrigation fluids, and suppositories.Amniocentesis: Percutaneous transabdominal puncture of the uterus during pregnancy to obtain amniotic fluid. It is commonly used for fetal karyotype determination in order to diagnose abnormal fetal conditions.