Mr. Peter Thompson:

It is on best dates. Best dates would be determined really by ultrasound scan in the first trimester. The best way to date a pregnancy is between 11 and 14 weeks by an ultrasound scan. If that is not possible, the dating becomes less and less accurate as time goes by. Effectively, I explain it to my patients as follows. With a small child, it is possible to tell roughly how old he or she is by how tall he or she is. That is the same up until approximately 24 weeks; we can determine how old a foetus is by its size. However, the standard deviation broadens quite significantly after that time. It would not be possible to tell the ages of those of us in this room by our heights. Later in gestation, it is necessary to do something else. Up until 24 weeks, the best way to do it is by ultrasound. In the UK it is by ultrasound scan.