At 28 weeks, the average size of the lateral ventricles in a baby's brain is around 6-9mm. Your baby's measurements of 7mm and 8mm fall within this range, which is generally considered normal. However, it's important to note that isolated measurements of ventricular size may not provide a complete picture of brain development. If there are no other concerning factors or abnormalities detected during your prenatal ultrasounds, it is likely that your baby's brain development is progressing normally. It is always recommended to discuss any concerns with your healthcare provider who can provide personalized guidance based on your specific situation.