There is no sensation at all from the ultrasound waves.A short history of the development of ultrasound in pregnancy can be found in the History pages.b) The Biparietal diameter (BPD) The diameter between the 2 sides of the head. It increases from about 2.4 cm at 13 weeks to about 9.5 cm at term.
Fetal body measurements reflect the gestational age of the fetus. In patients with uncertain last menstrual periods, such measurements must be made as early as possible in pregnancy to arrive at a correct dating for the patient. In the latter part of pregnancy measuring body parameters will allow assessment of the size and growth of the fetus and will greatly assist in the diagnosis and management of intrauterine growth retardation (IUGR).
Missed abortions and blighted ovum will usually give typical pictures of a deformed gestational sac and absence of fetal poles or heart beat.
Fetal heart rate tends to vary with gestational age in the very early parts of pregnancy.
Many women do not ovulate at around day 14, so findings after a single scan should always be interpreted with caution.
The diagnosis of missed abortion is usually made by serial ultrasound scans demonstrating lack of gestational development.