This is a scan between weeks 11-13+6. This scan can almost always be carried out transabdominally. There are number of benefits:
• To date the pregnancy accurately.
• To assess the risks of Down syndrome and other chromosomal abnormalities: Each woman will be given an estimate of her individual risk for this pregnancy. This is calculated by taking into account the age of the mother and the scan findings of nuchal translucency thickness, nasal bone, leakiness in the tricuspid valve and fetal abnormalities. Parents will receive full counseling concerning the significance of these risks and the various options for further testing.
• To diagnose multiple pregnancy: Approximately 2% of natural conception and 10% of assisted conceptions result in multiple pregnancy. Ultrasound scanning can determine chorionicity at this stage which is important for managing these pregnancies.
• To diagnose certain major fetal abnormalities:
Major abnormalities may be visible at this gestation but a mid trimester anomaly scan is essential.
• To diagnose early pregnancy failure: Couples will receive detailed counseling as to the possible causes of this problem and the options for subsequent measures.
Fetal anomaly scan 18-23 weeks (Mid-trimester anomaly scan/MTAS or Targeted Imaging for Fetal Anomalies/TIFA)
Even if the NT Scan is “normal”, it is important for all pregnant women to undergo this scan. This is a detailed scan during which each part of the fetal body is examined. Special attention is paid to the head, brain, face, spine, heart, stomach, bowel, kidneys and limbs. If any abnormalities are detected the significance of the findings will be discussed by the specialist in Fetal Medicine. If necessary, further counseling from other specialists will also be offered to the couple to discuss the postnatal course of the detected abnormality so that they can make an informed choice. At the time of the anomaly scan two other important parameters are assessed in the mother which help in predicting pregnancy complications like preeclampsia, fetal growth restriction and preterm birth. These parameters are the maternal uterine artery Doppler studies and the cervical length assessment.
• Uterine artery Doppler is a good predictor for maternal preeclampsia and fetal growth restriction. Although a more effective screening test in high risk pregnancies, it is still indicated in the ‘low risk’ population also as majority of these problems occur when we are suspecting the least.
• Cervical screening in the midtrimester of pregnancy has shown to be an effective way of filtering out the women who are more likely to have preterm delivery before 34 weeks. These women who test ‘screen positive’ (preferred term) or ‘high risk’ may be offered further surveillance and appropriate intervention to optimize the outcome.
Fetal wellbeing scan with Fetal Dopplers (Umbilical, Middle cerebral, and Ductus Venosus)
An ultrasound scan assesses fetal growth and wellbeing at 30-32 weeks of pregnancy. This scan aims to determine the growth and health of the fetus by:
• Measurement of the size of the fetal head, abdomen,thigh bone and calculation of an estimate of fetal weight.
• Examination of the movement of the fetus.
• Evaluation of the placental position and appearance.
• Measurement of the amount of amniotic fluid.
• Assessment of blood flow to the placenta and fetus by colour Doppler ultrasound.
Fetal growth is of paramount importance in the latter half of pregnancy and if growth restriction remains undetected, the outcome of the pregnancy can be adversely affected. A third trimester growth assessment helps us reassure most women about the satisfactory interval growth of the fetus and in the few cases where fetal growth is suboptimal, appropriate surveillance methods can be instituted.
Monitoring for Fetal Growth Restrictions
Fetal Doppler has proved to be a very good predictor of in-utero compromise of the growth restricted fetus. Doppler of various fetal blood vessels like, the umbilical artery and vein, the middle cerebral artery, the ,Ductus Venosus are used to decide the time of delivery of such a fetus since the postnatal diagnosis for such babies is still largely dependent on the gestational age at the time of birth. It has been proven beyond doubt that adequate surveillance and timely delivery are the two most important factors that can help in optimising perinatal outcome in cases with fetal growth restriction. Colour Doppler is an excellent tool to follow up fetuses with poor growth and decide on the best time for delivery.
The Middle cerebral artery Doppler is an invaluable, non-invasive tool to detect and quantify fetal anemia. This starts to become abnormal much before the fetus develops complications of anemia such as hydrops. It is now possible to diagnose fetal anemia with certainity by non-invasive colour Doppler study alone and plan for fetal therapy in the form of intrauterine blood transfusion with excellent results in terms of perinatal outcome – better maturity and neurodevelopmental outcome for the fetus suffering from anemia before birth. In a rhesus negative mother who has antibodies to the rhesus antigen, early referral and close surveillance with fetal MCA Doppler is vital in order to defect fetal anemia early in order to optimize the outcome.