Baby dating ultrasound
The point of the scan is to work out how many weeks pregnant you are. Ectopic pregnancy and miscarriage: diagnosis and initial management CG 154. You may have calculated the length of your pregnancy from the first day of your last menstrual period (LMP).
If your cycle is irregular or you've recently been on the pill, working out the date from your LMP probably won't work. If the scan date differs from your period dates, the scan date will be used.
Although the dating of pregnancy accuracy takes a dive, still ultrasound is indispensable for the other uses.
In early pregnancy, the measurement from the top of the fetal head to the bottom of the pelvis is called the “Crown-Rump” length.
Of all of these uses, dating the pregnancy is the most common reason to use ultrasound, particularly when the expectant mother cannot remember the date of her last period (as in breast-feeding or irregular cycles).
Even when the last period is known, ultrasound is reassuring to demonstrate adequate growth, especially when there’s a risk of delayed growth, as in hypertension or smoking, or if there’s the risk of exaggerated growth, as in gestational diabetes.
The scan will usually show that your pregnancy is less far along than suggested by your LMP.