<P> Other potential causes can include folate antimetabolites (such as methotrexate), maternal diabetes, maternal obesity, mycotoxins in contaminated corn meal, arsenic, hyperthermia in early development, and radiation . Studies have shown that both maternal cigarette smoking and maternal exposure to secondhand smoke increased the risk for neural tube defects in offspring . A mechanism by which maternal exposure to cigarette smoke could increase NTD risk in offspring is suggested by several studies that show an association between cigarette smoking and elevations of homocysteine levels . The study suggests that cigarette smoke, including secondhand exposure, is not only hazardous to the mother, but may also interfere with neural tube closure in the developing embryo . All of the above may act by interference with some aspect of normal folic acid metabolism and folate linked methylation related cellular processes as there are multiple genes of this type associated with neural tube defects . </P> <P> Folic acid supplementation reduces the prevalence of neural tube defects by approximately 70% of neural tube defects indicating that 30% are not folate - dependent and are due to some cause other than alterations of methylation patterns . Multiple other genes related to neural tube defects exist which are candidates for folate insensitive neural tube defects . There are also several syndromes such as Meckel syndrome, and Triploid Syndrome which are frequently accompanied by neural tube defects that are assumed to be unrelated to folate metabolism </P> <P> Tests for neural tube defects include ultrasound examination and measurement of maternal serum alpha - fetoprotein (MSAFP). Amniotic fluid alpha - fetoprotein (AFAFP) and amniotic fluid acetylcholinesterase (AFAChE) tests are also used to confirming if ultrasound screening indicates a positive risk . Often, these defects are apparent at birth, but acute defects may not be diagnosed until much later in life . An elevated MSAFP measured at 16--18 weeks gestation is a good predictor of open neural tube defects, however the test has a very high false positive rate, (2% of all women tested in Ontario, Canada between 1993 and 2000 tested positive without having an open neural tube defect, although 5% is the commonly quoted result worldwide) and only a portion of neural tube defects are detected by this screen test (73% in the same Ontario study). MSAFP screening combined with routine ultrasonography has the best detection rate although detection by ultrasonography is dependent on operator training and the quality of the equipment . </P> <P> In 1996, the United States Food and Drug Administration published regulations requiring the addition of folic acid to enriched breads, cereals, flour and other grain products . It is important to note that during the first four weeks of pregnancy (when most women do not even realize that they are pregnant), adequate folate intake is essential for proper operation of the neurulation process . Therefore, women who could become pregnant are advised to eat foods fortified with folic acid or take supplements in addition to eating folate - rich foods to reduce the risks of serious birth defects . In Canada, mandatory fortification of selected foods with folic acid has been shown to reduce the incidence of neural tube defects by 46% . </P>

When can a neural tube defect be detected