What is The Schedule for Ultrasound
There is no hard and fast rule as to the number of scans a woman should have during her pregnancy. A scan is ordered when an abnormality is suspected on clinical grounds. Otherwise a scan is generally booked at about 7 weeks to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown-rump length for dating.
A second scan is performed at 18 to 20 weeks mainly to look for congenital malformations, when the fetus is large enough for an accurate survey of the fetal anatomy. multiple pregnancies can be firmly diagnosed and dates and growth can also be assessed. Placental position is also determined. Further scans may be necessary if abnormalities are suspected.
Many centers are now performing an earlier screening scan at around 11-14 weeks to measure the fetal nuchal translucency and to evaluate the fetal nasal bone (and more recently, to detect tricuspid regurgitation) to aid in the diagnosis of Down Syndrome. Some centers will do blood test biochemical screening at the same visit.
Further scans may sometimes be done at around 32 weeks or later to evaluate fetal size (to estimate the fetal weight) and assess fetal growth. Or to follow up on possible abnormalities seen at an earlier scan. Placental position is further verified. The most common reason for having more scans in the later part of pregnancy is fetal growth retardation. Doppler scans may also be necessary in that situation.
The total number of scans will vary depending on whether a previous scan has detected certain abnormalities that require follow-up assessment. What is often referred to as a Level II scan merely indicates a "targeted" examination where it is done when an indication is present or when an abnormality is suspected in a previous examination. In fact professional bodies such as the American Institute of Ultrasound in Medicine does not endorse or encourage the use of these terms. A more "thorough" examination is usually done at an a perinatal center or specialised clinic where more expertise and better equipments may be present.
One should not dwell too much on the definitions or guidelines for a level II ultrasound scan. The prenatal sonologist should always try very hard to look for and assess any abnormality that may be present in the fetus. It is not very meaningful to be talking about level III or even level IV scans.
That a pregnancy should be scanned at 18 to 20 weeks as a rule is gradually becoming a matter of routine practice.