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ANTENATAL SCREENING 
 

Antenatal screening

Screening in pregnancy implies strategies for the detection of disorders at a stage at which the patient has no signs or symptoms.

Routine at first antenatal visit, before 20 weeks

Blood group (ABO) and Rh testing Red cell antibody screen Rubella serology Hepatitis B surface antigen (HbsAg) Syphilis serology Urine MCS

Routine at 28 weeks

Blood group and red cell antibody screen

Routine at 30 ' 36 weeks

FBC Urine MCS Genital swab for group B streptococcus

If high risk for infection

Urine or endocervical swab for chlamydia PCR Endocervical swab for gonococcal culture HIV serology Herpes simplex virus culture or PCR of cervical swabs Hepatitis C serology 

Additional tests, if appropriate

Down's syndrome antenatal screen AFP (neural tube defect screen) Cervical cytology Toxoplasma serology Varicella zoster serology


NOTES

FBC

All pregnant women should have a haemoglobin concentration and haematocrit determination early in each pregnancy since anaemia is the most common complication of pregnancy, usually due to iron deficiency, but may also be due to folate deficiency, rarely blood loss and vit. B12 deficiency and disorders of haemoglobin synthesis.
 
Blood group (ABO) and Rh testing

It is important to know a pregnant woman's ABO type so that steps can be taken to assure that type specific blood is available for transfusion should she have an uncommon type. ABO type information is also important if ABO incompatibility is being considered in the diagnosis of neonatal jaundice. There is a 70 ' 75% chance that the foetus being carried by an Rh negative women is Rh positive. Greater than 99% of Rh isoimmunisation can be prevented with the appropriate use of Rh immunoglobulin (RhoGam) administered to the mother at 28 weeks gestation. After delivery, the infant of an Rh negative mother should have Rh type determination, and if positive, additional Rh immunoglobulin should be administered to the mother within 72 hours of delivery.

Red cell antibody screening
Maternal sensitization to certain red blood cell antigens present in the foetus puts the foetus at risk for erythroblastosis foetalis and the baby at risk of haemolytic disease of the newborn. Rh antibody (anti-D) is most commonly responsible for this problem; however, maternal IgG antibodies to several red cells antigens including C, c, E, e, Kell, Duffy, and Kid may also put the foetus and newborn at risk. A woman can develop this type of isoimmunisation through a previous pregnancy or via previous exposure to blood products.

Rubella serology
Primary maternal rubella infection in the first trimester of pregnancy carries devastating consequences for the foetus, especially in the first 8 weeks of pregnancy. Although infection is symptomatic in most mothers, asymptomatic infection does occur. Rubella serology is unnecessary in those with proven immunity to rubella virus. Non-immune women should be offered rubella vaccination after birth.

HbsAg
There is up to an 80% chance of transmission of hepatitis B virus vertically from the mother to the child if the mother is a hepatitis B carrier. Post exposure prophylaxis is possible with hepatitis B vaccine and immunoglobulin. Therefore screening for hepatitis B surface antigen (HbsAg) in pregnant women would be justified in areas of high prevalence. However, even in areas of low prevalence, there is movement towards the implementation of antenatal screening, so that infants of mothers who are hepatitis B positive may be protected by prophylaxis.

Syphilis serology
Any stage of syphilis during pregnancy can result in an infected foetus. Congenital syphilis occurs when the treponemes cross the placenta after the 16 ' 18th week of gestation. In rare circumstances, infection may occur from contact with an infectious lesion during passage through the birth canal. Treatment is important ' to eradicate maternal infection and to prevent congenital syphilis.

Urine MCS
All pregnant women should be screened for asymptomatic bacteriuria early in pregnancy. Those with documented UTI should be treated, and test for cure should be done following antibiotic therapy.

Genital swab for group B streptococcus (GBS) Advocated in some centres. Others recommend administering intrapartum ampicillin for defined risk factors such as preterm birth before 35 weeks gestation, membranes ruptured > 18 hours before delivery, maternal temperature ≥ 38'C, GBS colonisation or bacteriuria ever detected.

HIV antibodies

The rate of vertical transmission of HIV varies from 15 ' 50%. The question of whether to or not to screen for HIV antibodies in pregnant women is a difficult one as it involves ethical and legal considerations, even though antiretroviral agents such as AZT or nevirapine have been shown to significantly reduce transmission of HIV to the infant.

Herpes simplex virus ( HSV) culture
Genital herpes in the mother is an obvious predisposing factor for the acquisition of neonatal HSV infection. The majority of infected infants are born to mothers who have no signs or symptoms of active infection at the time of delivery but in whom asymptomatic shedding of HSV occurs. The indications for an elective caesarian section are controversial but the majority of obstetricians would recommend it in the presence of a florid primary maternal infection at the time of labour. Recurrence of genital herpes is not an indication for caesarean section. Since shedding of HSV from the genital tract is common and neonatal HSV is rare, there is no justification for the routine culture for HSV shedding in pregnant women. Viral cultures however should be performed when a woman has a suspicious lesion to confirm the diagnosis of HSV.

Down's syndrome antenatal screen
Using the maternal serum levels of AFP, oestriol and hCG, the Down's syndrome detection rate is about 60%. These markers are also useful in the detection of other rare chromosome disorders, e.g. trisomy 18. Currently, women more than 35 years are offered amniocentesis for maternal age related risks associated with chromosomal abnormalities. The 'triple test' is hence offered to women less than 35 years and is performed between 15 ' 20 weeks gestation. An ultrasound is recommended in patients with a positive result to confirm gestational age. If gestational age is confirmed by ultrasound, then genetic counselling and amniocentesis can be offered.

Alpha-foetoprotein (neural tube defects)
More than 90% of NTDs occur in pregnancies in which no previous increased risk has been identified. Several developmental abnormalities are associated with an increased maternal AFP; these include open neural tube defects (spina bidida and anencepahy), and ventral wall defects. Screening is most accurate when performed between 15 and 20 weeks gestation. When positive results occur, the next step is an ultrasound to confirm gestational age and to identify multiple pregnancy, foetal death or structural birth defects. If ultrasound provides no explanation for the positive result, then the maternal AFP may be repeated, genetic counselling be offered, and amniocentesis considered.

Cervical cytology
Documented normal cervical cytology within the preceding 18 months may be used to delay repeat screening if there is no clinical indication for another PAP smear.

Toxoplasma serology
Screening is not recommended as a routine. Screening for toxoplasma may be warranted in pregnant women who are cat owners.

Varicella-zoster serology
Approximately 95% of adults are immune as the result of natural infection. Varicella infection is extremely rare in pregnancy. Pregnant women with no history of chickenpox (VZV infection) who is exposed to varicella should have varicella-zoster serology performed within 24 ' 48 hours. If the patient is known to be seronegative, VZ-immunoglobulin should be administered within 4 to 6 days to prevent maternal and congenital infection.
 
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