Becoming more active is only part of what’s happening with your baby at week 16. The baby’s circulatory and urinary systems are functioning at a more advanced stage.
Your baby’s head also appears more “normal” as the eyes and ears have settled into their permanent position on the head. Your baby’s head is also becoming more erect and not angled forward as it had been for the first few months.
-During the 16-week scan, you can expect to see a baby that is fully formed but just very small.
Reference :-
1-https://www.healthline.com/health/pregnancy/16-weeks-pregnant#your-baby
2-https://www.healthline.com/health/pregnancy/16-week-ultrasound.
3-https://www.whattoexpect.com/pregnancy/week-by-week/week-16.aspx