Screening for fetal complications

CONFIRMATION OF FETAL VIABILITY

All women should be offered a 'dating' scan. This is best performed between 10 and 13 weeks' gestation and the crown-rump length measured when the fetus is in a neutral position (i.e. not curled up or hyperextended). Current evidence shows that the estimated day of delivery predicted by ultrasound at this gestation will reduce the need for induction of labour at 41 weeks when compared with the due date predicted by the last menstrual period. In addition, a dating scan will improve the reliability of serum screening for Down's syndrome, diagnose multiple pregnancy and allow accurate determination of chorion-icity and diagnose up to 80% of major fetal abnormalities. Women who present after 14 weeks' gestation should be offered a dating scan by ultrasound assessment of the biparietal diameter or head circumference.

SCREENING FOR DOWN'S SYNDROME

Current recommendations from the National Screening Committee and the National Institute for Clinical Excellence advocate that Down's screening programmes should detect 60% of affected cases for a 5% false positive rate. By 2007 the detection rate should be 75% for a 3% false positive rate. These performance measures should be age standardized and based on a cut-off of 1/250 at term. There are numerous screening strategies operational in the UK at the present time using either first trimester ultrasound markers (nuchal translucency) or maternal serum markers

(alpha-fetoprotein, oestriol, free-beta hCG, inhibin-A and pregnancy associated plasma protein A) in either the first or second trimester. Some programmes use a combination of both serum and ultrasound markers. To achieve the 2007 targets, it is likely that combination screening will be required. Because screening for Down's syndrome is a complex issue, health-care professionals must have a clear understanding of the options available to their patients. Unbiased, evidence-based information must be given to the woman at the beginning of the pregnancy so that she has time to consider whether to opt for screening and the opportunity to clarify any areas of confusion before the deadline for the test passes. Following a 'screen positive' result the woman needs careful counselling to explain the test result does not mean the fetus has Down's syndrome and to explain the options for further testing by either chorion villus sampling or amniocentesis. Apositive screen test does not mean further testing is mandatory. Likewise, a woman with a 'screen negative' result must understand thefetus may still haveDown's syndrome(see 'Fetal medicine in clinical practice').

SCREENING FOR STRUCTURAL ABNORMALITIES

The identification of fetal structural abnormalities allows the opportunity for in utero therapy, planning for delivery, for example, when the fetus has major congenital heart disease, parental preparation and the option of termination of pregnancy should a severe problem be diagnosed. Major structural anomalies are present in about 3% of fetuses screened at 20 weeks' gestation. Detection rates vary depending on the system examined, skill of the operator, timeallowed for thescan and quality of theultra-sound equipment. Follow-up data is important to audit the quality of the service. Women must appreciate the limitations of such scans. Local detection rates of various anomalies such as spina bifida, heart disease, facial clefting and the like should be made available. Written information should be given to women early in pregnancy explaining thenatureand purposeof such scans highlighting conditions that are not detected such as cerebral palsy and many genetic conditions. It is important to appreciate that the fetal anomaly scan is a screening test which women should opt for rather than have as a routine part of antenatal care without appropriate counselling (see 'Fetal medicine in clinical practice').

SCREENING FOR FETAL WELL-BEING

Each antenatal clinic attendance allows the opportunity to screen for fetal well-being. Auscultation for the fetal heart will confirm that the fetus is alive and can usually be detected from about 14 weeks of gestation. While hearing the fetal heart may be reassuring there is no evidence of a clinical or predictive value. Likewise there is no evidence to support the use of routine cardiotocog-raphy in uncomplicated pregnancies. Physical examination of the abdomen by inspection and palpation will identify approximately 30% of small-for-gestational-age fetuses [6]. Measurement of the symphysio-fundal height in centimetres starting at the uterine fundus and ending on the fixed point of the symphysis pubis has a sensitivity and specificity of approximately 27 and 88%, respectively, although serial measurements may improve accuracy. Customized growth charts make adjustments for maternal height, weight, ethnicity and parity. Their use increases the antenatal detection of small-for-gestational-age fetuses and result in fewer unnecessary hospital admissions. While the evidence for the benefits of plotting serial symphysio-fundal height measurements is limited, it is recommended that women are offered estimation of fetal size at each antenatal visit and when there is concern, referred for formal ultrasound assessment. Traditionally, women have been advised to note the frequency of fetal movements in the third trimester. Although the evidence does not support formal counting of fetal movements to reduce the incidence of late fetal death, women who notice a reduction of fetal movements should contact their local hospital for further advice.

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