Ultrasound scan is currently considered to be a safe, non-invasive, accurate and cost-effective investigation in the fetus. It has progressively become a valuable obstetric tool and plays an important role in the care of every pregnant woman.
The gestational sac can be visualized as early as four and a half weeks of gestation and the yolk sac at about five weeks. The embryo can be observed and measured by about five and a half weeks. Ultrasound can also very importantly confirm the site of the pregnancy is within the cavity of the uterus. Many structural abnormalities in the fetus can be reliably diagnosed by an ultrasound scan, and these can usually be made before 20 weeks. Doppler ultrasound is presently most widely employed in the detection of fetal cardiac pulsations and pulsations in the various fetal blood vessels.
3/4-D ultrasound can give us visual 3/4 dimensional image of the scan.
The transducer takes a series of images, thin slices, of the subject, and the computer processes these images and presents them as a 3/4 dimensional image. A good 3/4-D image is often very impressive to the parents.
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. 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 and can 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.
It has been over 40 years since ultrasound was first used on pregnant women. Unlike X-rays, ionizing irradiation is not present. Current evidence indicates that diagnostic ultrasound is safe for the unborn child.