Screening tests in pregnancy

Diabetes screening

 

• Human placental lactogen and cortisol are insulin antagonists which can result in the mother developing marked insulin resistance
• As glucose crosses the placenta by facilitated diffusion levels in the fetus mirror that of the mother
• 1-2% of pregnant women develop gestational diabetes
• risk factors
  • obesity
  • family hx of diabetes
  • previous large baby >4.5kg
  • previous unexplained still birth
  • previous congenital abnormality
  • women with previous diabetes will require increase insulin during pregnancy

 

Fetal complications

• increased risk of miscarriage
• congenital abnormality
• neural tube defects
• congenital heart disease
• spinal abnormalities such as caudal regression syndrome
• fetal macrosomia can result in birth trauma, shoulder dystocia and hypoxic damage

 

Neonatal complications

• Hypoglycaemia
• Respiratory distress syndrome
• Hypomagnesium
• Polycythaemia

 

Screening

• Urinary glucose is unreliable
• Random blood glucose test –should be <6mmol/L preprandial
• Oral glucose tolerance test – measurement of plasma glucose concentration 2 hours after a 75g oral glucose load. The patient should fast 8-12 hours prior to test.
• The 2 hr level after 75g glucose should be <7.8mmol/L

 

 

 

 

 

Down’s screening

 

• Prevalence of Down’s increases with maternal age
• Women over the age of 35 are offered diagnostic testing for Down’s

 

Screening via maternal serum levels

• AFP and human chorionic gonadotrophin are of fetal origin
• In Down’s pregnancies, AFP is decreased whilst hCG is increased during the 2^nd trimester
• Not diagnostic but identifies a group of women with a high risk >1:250 who can then be offered diagnostic tests such as amniocentesis
• In the 1^st trimester, pregnancy associated plasma protein (PAPP-A) is lowered in Down’s whilst hCG is elevated
• The ‘triple’ test is done around 16 weeks and measures;
  • Alpha-fetoprotein
  • hCG
  • unconjugated oestriol (low in Down’s pregnancy)

 

Ultrasound screening

• Nuchal translucency – measurement of the collection of fluid behind the neck of the fetus at 10-13 weeks.
• This is increased in aneuploidy in the 1^st trimester
• Other ultrasound abnormalities
  • Bradycephaly and ventriculomegaly in the skull and brain
  • Cardiac defects – echogenic chordae tendineae in the heart
  • Echogenic bowl
  • Mild hydronephrosis
  • Short femur
  • Abnormal hands and feet
• Fetus may also be small for gestational age
• Amniocentesis
  • Performed after 15 weeks
  • Under ultrasound guidance a needle is inserted into the amniotic cavity and amniotic fluid is sampled
  • Chance of miscarriage is increased by 1%
• Chorionic villus sampling
  • Sampling of the trophoblast of the placenta
  • Can be done after 11 weeks
  • Possibly slightly higher risk of miscarriage

 

 

 

 

 

 

 

 

 

 

 

National/East Anglian Neonatal screening program

 

• Heel prick test done day 5-8 after birth
• Tests for phenylketonuria and congenital hypothyroidism
• In east anglia high risk babies are also screened for cystic fibrosis
• Phenylketonuria
  • Deficiency in phenylalanine hydroxylase required to metabolise phenylalanine to tyrosine. Excessive phenylalanine can result in mental retardation and seizures
• Congenital hypothyroidism
  • Leads to cretinism – growth failure and retardation

 

 

Rhesus D+ Haemolytic Disease

 

• Does not affect the first pregnancy
• If Rhesus negative mother has been previously sensitised to Rhesus D positive RBC then IgG antibodies can develop and cross the placenta in subsequent pregnancies
• This can lead to fetal haemolysis if the baby is rhesus positive which may necessitate fetal transfusion
• Potential sensitising events for rhesus disease
  • Miscarriage
  • Termination of pregnancy
  • Antepartum haemorrhage
  • Invasive prenatal testing
  • Delivery
• The two other rhesus antigens C and E may also be associated with haemolytic disease of the new born
• Prevention is sensitisation is by administering anti-D immunoglobulin within 72hr of event
• If a women is Rhesus negative then monitor atypical antibody levels during pregnancy. An increase to >10iu/ml requires fetal medicine review to look for signs of fetal anaemia
• The recommended dose is 500iu after 20weeks and 250iu before 20weeks gestation
• Appropriate dose adjustment can be done via the Kleihauer test which measure the amount of fetal RBC present in the maternal blood (due to their ability to resist denaturaation by alcohol or acid)