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Factor V

A protein in the blood that promotes clotting of the blood by accelerating the activation of prothrombin to thrombin. Activated factor V is inactivated by activated protein C.

Factor V Leiden Mutation (activated protein C resistance)

A genetic mutation in the factor V gene that makes the activated factor V protein resistant to inactivation by protein C. The increased activity of factor V in the blood leads to a higher risk of forming a blood clot (thrombophilia) . The factor V Leiden mutation has a prevalence of  5–9% in the general population.

Factor V Leiden mutation in the mother has been associated with an  increased the risk of early first trimester recurrent pregnancy loss and stillbirths.

Femur length (FL)

The distance from end to end of the thigh bone (the longest bone  in the body). The distance is measured by sonogram along the femur shaft (diaphysis) and should NOT include the DFE (distal femoral epiphysis) a secondary bone forming center.

To obtain the appropriate plane for measurement of the FL align the ultrasound beam perpendicular the bone.

DFE = Distal femoral epiphysis


A protein found inside cells that stores iron in a nontoxic bioavailable form.

Fetal fibronectin (fFN)

Fetal fibronectin (fFN) is a glycoprotein that acts like "glue" holding the fetal sac to the uterine lining during pregnancy. It can normally be found in the cervicovaginal secretions of women up to 22 weeks of gestation. However, the presence of fetal fibronectin in cervicovaginal secretions between 24 and 34 completed weeks of gestation is reported to be associated with preterm delivery.

Pregnant women between 24 weeks, 0 days and 34 weeks, 6 days gestation with signs and symptoms of early preterm labor, intact amniotic membranes and minimal cervical dilation (<3cm) may be tested for the presence of fetal fibronectin by swabbing secretions from  the posterior fornix (the vaginal area under the cervix) .  The test is reported as positive or negative.

The probability that a pregnant woman will deliver prematurely after a positive test is only 40 to 60 percent. However, if the test is negative there is a 99.5 percent chance the pregnant woman will NOT deliver within seven days and a 92 percent chance that she will NOT deliver before 37 weeks. The test is, therefore, much more valuable for predicting who is unlikely to deliver.


A human conceptus from 70 days' gestational age until delivery

Fetal viability

The capacity for sustained survival outside the uterus as determined by the judgment of the responsible attending physician.

The Textbook of Neonatal Resuscitation, 4th edition, advocates that no resuscitation of newborns less than 23 weeks’ gestational age and/or 400 g birth weight is appropriate [1]. Newborns with malformations incompatible with life such as renal agenesis, anencephaly, trisomy 13 , or trisomy 18  are not considered viable regardless of the gestational age.

1. American Academy of Pediatrics. Special considerations. In: Braner D, Kattwinkel J, Denson S, Zaichkin J, eds. Textbook of Neonatal Resuscitation. 4th ed. Elk Grove Village, IL: 2000:7–19

Fetal Warfarin Syndrome

A group of birth defects associated with exposure of the developing fetus to warfarin (Coumadin®) an oral anticoagulant used to prevent blood clots. The defects include nasal hypoplasia, stippled epiphyses, limb deformities, mental retardation, and seizures. Hall et al. (1980) placed the critical period for warfarin effects on the fetus as 6 to 9 weeks following conception.

Hall JG , et al. Maternal and fetal sequelae of anticoagulation during pregnancy.Am J Med. 1980 ;68:122-40.  PMID: 6985765

Fifth Disease. See Erythema infectiosum

Fibroid (leiomyoma, myoma)

A non-cancerous (benign) growth of fibrous tissue and muscle fibers. Fibroids are often found in the uterus, and occur in about 30% of women over the age of 30.

Follicle stimulating hormone (FSH)

A hormone (a chemical) produced by the pituitary gland in the brain that stimulates the growth of eggs in the ovaries and the production of sperm in the testes.

An elevated FSH suggests partial or complete ovarian failure.

Fontanel ("soft spot")

A membranous gap between the bones of the developing skull.

The newborn normally has six fontanels the anterior, posterior, two mastoid, and two sphenoid fontanels. The most prominent fontanel is the diamond-shaped anterior fontanel which is closed in 96 percent of infants by 24 months of age.

Fragile X Syndrome (Martin-Bell syndrome)

A disorder affecting males and females characterized by mental retardation. Males , large ears and jaws, macroorchidism in males (see illustration). In the U.S. fragile X syndrome affects approximately 1 in 4000 males and 1 in 8000 females and is a common cause of autistic-like behaviors.

Fragile X syndrome is caused by an expansion mutation that prevents the normal expression of the the Fragile X mental retardation 1 (FMR1) gene located on the X chromosome.

Near the beginning of the (FMR1) gene there is normally a series of repeating nucleotide DNA bases Cytosine, Guanine, Guanine (CGG). Usually there are about 30 repeats of CGG. However some individuals have from 55 to 200 CGG repeats (the pre-mutation) which have a predilection to expand further as the gene is passed from one generation to the next through the mother. The pre-mutation does not usually expand in size when passed from father to daughter.  Persons with the full mutation have more than 200 of the CGG repeats and show symptoms of fragile X syndrome.

A positive family history in a proband with developmental delay should prompt consideration of genetic testing of the FMR1 gene. The American College of Medical Genetics recommends testing for all males and females with mental retardation of unknown etiology

Fraternal twins (dizygotic twins)

Two offspring created when two separate eggs are fertilized by separate sperm during the same menstrual cycle. The two products of conception develop as two separate embryos, and each embryo develops its own membranes and placenta.

Fundal height

The distance from the top of the pubic bone to the top of the pregnant uterus (fundus) in centimeters. The measurement is used to approximate fetal age and growth.

The measurement in centimeters gives an estimate of the gestational age in weeks from 16 to 38 weeks (+/- 3 centimeters). The measurement may not coincide with the gestational age if there is a growth disturbance , large fibroids, or abnormalities of the amniotic fluid.


The uppermost part of the uterus.



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