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I have gone through your reports (attachment removed to protect patient identity) in toto.
At 26 weeks period of gestation, the overall growth of the fetus is fine. But the concern here is hypoplastic nasal bone seen in the second trimester TIFFA (targeted imaging for fetal anomalies) scan. One has to understand that nasal bone hypoplasia (underdevelopment of nasal bone) have been designated as a single marker for Down's syndrome with a likelihood ratio of 81. There are other soft markers for Down's syndrome which indicate towards the likelihood namely:-
- Nuchal fold thickness more than 6 mm; likelihood ratio is 17:1.
- Hypoplastic nasal bone has emerged in recent years as a particularly strong marker. The absence of a nasal bone has a likelihood ratio of 83:1.
- Echogenic intracardiac focus; likelihood ratio is 1.5.
- Echogenic bowel; likelihood ratio is 6.1.
- Shortened humerus; likelihood ratio is 7.5.
- Shortened femur; likelihood ratio is 2.7.
- Single umbilical artery, only considered as a soft marker if other abnormalities are present. In an otherwise normal second trimester ultrasound, it is not a marker of aneuploidy (abnormal number of a chromosome).
- Mild pyelectasis (dilated renal pelvis), a weak association with aneuploidy.
Also what can be done in second-trimester screening for Down's syndrome is the quadruple screening, which is ideally done between 11-14 weeks of the period of gestation. It involves serological markers MSS2.
- Maternal free beta hCG (human chorionic gonadotropin): Higher than chromosomally normal fetuses.
- Inhibin A: Higher than chromosomally normal fetuses.
- AFP (alpha-fetoprotein): Lower than chromosomally normal fetuses.
- Unconjugated estriol (uE3): Lower than chromosomally normal fetuses.
In your case now there is only one marker present on TIFFA, but a strong marker indeed. So, amniocentesis (amniotic fluid test) is indicated under expert hands.
As 26 weeks, the chances of initiating preterm labor are fairly high as the baby is already 900+ g weight. But, considering that the AFI (amniotic fluid index) is high at 20, it can be attempted. Following amniocentesis, the fetal cells so obtained can be subjected to karyotyping studies (to check chromosome count), which would be conclusive if it shows Trisomy 21 pattern on chromosomal studies.
Now, presuming that the result is positive for DS, there is a legal hassle where pregnancy cannot be terminated beyond 20 weeks period of gestation as per MTP ACT (Medical Termination of Pregnancy Act). Only under special conditions where the child is either going to be harmful to the mother's health or affecting the mental status of the mother psychologically, with a legal intervention the termination can be undertaken with the opinion of two doctors plus one legal counselor.
The termination would be a vaginal route and chances of the post part hemorrhage at this period of gestation are very high and can be dangerous for the mother. So ideally, we do not advise at all. Hence, we prefer regular ANC (antenatal care) visits so that any anomalies are caught early in the antenatal period.
If you plan to deliver the child, it will take the course of normal pregnancy. One has to accept the fact that post birth, the child would have delayed milestones and maximum life expectancy is 17-18 years as recorded in literature. The scan undertaken has good resolution indeed, and they have undertaken a 4D scan for TIFFA. However, you may choose any diagnostic center or hospital for a scan to reconfirm the report. You may consult your gynecologist for the same. I would suggest an amniocentesis and karyotyping at the earliest.