Yes it is normal to have mouth open in 20 week ultrasound .Infact the fetus drinks the amniotic fluid and passes urine while in the womb.
This is very rare with incidence of one in 300000 births.I have not seen even a single case in my clinical practice.I dont think you should be bothered at all.If Foetal heart rate and all other parameters(foetal organ measurments) are normal you should not worry about these congenital abnormalities.Foetus drinks amniotic fluid which resulted in open mouth and i dont think anything to worry.
There are two types of Ultrasound Level 1 and level 2 .Level 2 ultrasounds are more detailed and can detect congenital abnormalities accurately upto the tune of 90% .
Cleft palate should not be missed on Ultrasound.A cleft palate can be diagnosed as early as 16 weeks.
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