When an ultrasound is performed, measurements of the head, abdomen, thigh, and amount of amniotic fluid are done.These measurements are computed automatically in the ultrasound machine’s software.For instance, say you normally have eight and nine pound babies.Then your baby at 37 weeks will have bigger measurements than babies destined to weigh seven or eight pounds at birth.Therefore, measurements taken later on, when babies begin to grow at different rates among pregnancies, yield increasingly inaccurate dating of pregnancy.Although the dating of pregnancy accuracy takes a dive, still ultrasound is indispensable for the other uses.The software has certain measurement scales based on data from large populations, and your baby’s measurements are put into this scale.In other words, by comparing your baby’s measurements to the data from this large collection of measurements, the ultrasound can then tell how far along your baby is.
Ask your GP if you want to have one anyway for extra reassurance, as she may be able to refer you.
However, the timing of your first scan will also depend on how your pregnancy is going, and where you live.
Your midwife may recommend an early scan at between six weeks and 10 weeks if you have experienced bleeding, pain, or problems in a previous pregnancy.
The computer in the ultrasound (ignorantly) lumps your bigger baby into the dates of babies that big in the “normal” population.
This may then indicate that your 37-week baby is two weeks overdue!Most babies, except in cases of early IUGR and deformities, grow about the same until 20 weeks or so.