Open dialogue. In a good open relationship with your OB you shouldn't have to insist on anything. If s/he is anything like the ob's i work with they would prefer for you to openly share any concern you have & are likely to do another ( spaced a bit to see if the growth is consistent) even without your request. If you raise a concern they would share the issue of timing & information they get from each study.
Agree with Dr. F. I'm sure your OB has offered you all the latest tests for chromosomal abnormalities because of the increased risk at your age & is monitoring your pregnancy closely. On growth charts & standardized tests, 25th%ile-75th%ile is the average range; 50th%ile is the mean. So, if length, weight & head circumference are growing along the 25th%ile, that's normal. The velocity of growth is what's important.