Ultrasound during pregnancy
With the advancing ultrasound technology, it is now possible to observe the complete development of the fetus since the time of missed period. It is important to do the first sonography at least after one week of the missed period to confirm that pregnancy is in the uterus. This is important because pregnancy outside the uterus in tube or elsewhere is a very risky condition for the female and may be even life threatening is it ruptures. If the pregnancy is in the uterus, its cardiac activity must be confirmed a week after that. Ideally it is good to monitor high risk pregnancies closely, at least every two weeks. But is this is not done at least a detailed scan at 11-13 weeks is absolutely essential. This scan is done to rule out chromosomal abnormalities. These abnormalities lead to mental retardation. It is important to do this scan so early in the pregnancy for two reasons.
- We know that these children are not capable of independent existence and so parents would have a chance of terminating the pregnancy if the fetus is diagnosed of such an abnormality.
- The signs that indicate the presence of such an abnormality are transient and may disappear or become less recognizable after 14 weeks.
The signs that would indicate the presence of a chromosomal abnormality are called chromosomal markers. Some markers are more important that others and these are known as major markers.
These are :
- Increased nuchal translucency.
- Short / absent nasal bone.
- abnormal ductus venosus flow.
1. Increased nuchal translucency:
This in very simple words may be called swelling on the back side of the neck. At this time the whole baby is hardly 5.5 cms in length and the swelling measures only 3 mm. It is a very sensitive measurement and can be erroneously measured if the section is slightly different or even if the fetus position is a little different. Therefore measuring this swelling (Nuchal translucency- NT) needs very good training, more practice and good equipments. If the fetus has this swelling, it has a very high risk of having either chromosomal abnormality or cardiac abnormality and therefore in these foetuses, it is essential to do chorion biopsy or amniocentesis to confirm the chromosomal status.
2. Short or absent nasal bone:
We have noticed that children with monogolism- trisomy 21- chromosomal abnormality have a very short nose. This can be noticed as early as 11-13 weeks of pregnancy also. Like NT, this is also a very small structure at this age of pregnancy and so needs practice and training to measure or see it correctly. Absent nasal bone also demands chorion biopsy or amniocentesis because the risk of chromosomal abnormality increases by 146 folds with absence of nasal bone.
3. Abnormal ductus venosus flow:
Ductus venosus is a very important vessel for transfer of oxygenated blood from mother to the fetus. At 11-13 weeks it is only 2 mm in diameter at its narrowest part. It normally has a definite flow pattern. Change in this flow pattern is also an indication of chromosomal or cardiac abnormality.
Some new markers of importance have come up in last few years. These are:
- Facio-maxillary frontal angle
- Tricuspid regurge
- Intracranial translucency
Faciomaxillary angle measures the flatness of the face. Mongol babies have flat face and this can be assessed on this scan at 11-13 weeks.
Tricuspid is the valve of the right side of the heart. Back flow from this valve is known as regurge. This indicates cardiac or chromosomal abnormalities.Intracranial lucency can diagnose defects in the spinal canal as early as 12 weeks.
There are other markers also which are not very indicative of chromosomal abnormalities but when more than one of those are present, have a predictive value.
These are club feet, omphalocoele, renal pyelactasis, echogenic focus in heart and echogenic bowel.
This scan if done correctly has 94% sensitivity for diagnosis of chromosomal abnormalities.
After this test the another essential scan is done between 18-22 weeks. This is a detailed structural scan. All the organs of the body become functional around 18 weeks, so this scan is very important. It consists of detailed study of fetal heart- fetal echocardiography, detailed study of fetal brain- fetal neurosonogram, etc. It also includes Doppler study for assessment of the risk of PIH-IUGR. This scan also screens for other chromosomal markers which present after 18 weeks. These are ventriculomegaly, choroid plexus cyst, gap between great toe and second toe- saddle gap, cleft lip, cleft palate, low set ears, etc. Spinal abnormalities are also all diagnosed at this scan. The advantage of diagnosing all these abnormalities antenatally, is that if the child is diagnosed of correctable abnormalities, this treatment can be planned as early as possible after birth and the complications can be prevented. Where as if the fetus is diagnosed of uncorrectable abnormalities with which, independent existence of the child is not possible, parents can be given an option of termination.
The third scan which must be done is at 30-32 weeks. This is a Doppler study. It is done to find out if the patient is going to develop hypertension, is there a risk that the fluid around the baby will decrease, is there a chance that the baby will have to be delivered early??? If the risk is proved, a closer follow up is done. Time of delivering the baby in high risk babies should be based on Doppler of the vessels of the brain and it oxygenation. Delay in the time of delivery after the oxygen supply to the baby’s brain is compromised leads to neurological and intellectual deterioration of the baby.