The proportions of your baby's body are becoming much closer to those of a newborn baby. Up until the third month, his head accounted for almost half of his overall length. Now his head, trunk, and legs each account for a third. At birth your baby's head will still be large in proportion compared to an adult's, but it will be just a quarter of his overall length.
Still very skinny, your baby is now starting to fill out more as fat reserves continue to be laid down. Up until this point most babies are approximately the same size and weight. Genetic and especially environmental forces increasingly come into play, influencing how quickly your baby can grow and whether his full growth potential is eventually reached.
Because different babies increasingly vary in size from this stage of pregnancy, ultrasound dating becomes less accurate at determining the number of weeks you are into your pregnancy. The best time to date the pregnancy is between 11 and 14 weeks, simply measuring top to bottom (the crown rump length) (see Your first scan). The 20-week scan (see Your 20-week Scan) also dates the pregnancy very accurately from the head and abdominal circumferences and the bone measurement in the leg.
If your first scan is at this late stage, it is only possible to estimate roughly the stage of pregnancy and age of the baby. It is never appropriate to change your due date if an earlier, more accurate scan has been performed.