Thank you for your query.
Only the complete ultrasound
report and assessment by your obstetrician can tell you the cause of decreased growth of the twin.
In twins, there is a condition called discordant twins, where if the weight differs by more than 25 %, chances of complications are very high.
Moreover, you cannot just guess the cause of this.
If the report says that the amniotic fluid
is less, simple dehydration
cannot cause this in early pregnancy
You need to consult your doctor for further information and guidance with your ultrasound report.
Meanwhile, taking lots of fluids and a nutritious diet is always beneficial in pregnancy.