Ultrasound scan is currently considered to be a safe, non-invasive, accurate and cost-effective investigation in the fetus.Ultrasound has no harmful side effects to you or your baby as it does not use radiation, as X-ray tests do. At any age of pregnancy ultrasound does not impose any harm and can be carried out safely.
Certain harmful effects of ultrasound in cells are observed in a laboratory but large studies reports that it does not cause any abnormalities in embryos and offsprings of animals and humans. Unconfirmed small studies unsubstantiated with larger studies from Europe reports ill-effects such as low birthweight, speech and hearing problems, brain damage and non-right-handedness. Many studies ore on-going to to be certain with the safety of ultrasound but no documented strong evidence has been reported so far.
The greatest risks arising from the use of ultrasound are the possible over- and under- diagnosis brought about by inadequately trained staff, often working in relative isolation and using poor equipment.
At 4 weeks ultrasound is usually done to confirm pregnancy and to confirm that the site of the pregnancy is within the cavity of the uterus.; abdominal ultrasound is sometimes unable to appreciate the fetus at this early, in case of suspected extra-uterine pregnancy a trans-vaginal ultrasound using a probe will be carried out. Gestational sac can be visualized as early as four and a half weeks of gestation and the yolk sac at about five weeks.
There is no hard and fast rule as to the number of ultrasound scans a pregnant woman should have. 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 ( routine practice) 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. (Many centers are now performing an earlier screening scan at around 11-14 weeks). Placental position is also determined.
Further scans may sometimes be done at around 32 weeks or later to evaluate fetal size and assess fetal growth, to follow up on possible abnormalities seen at an earlier scan,and to verify placental position.Some centers do another scan a week before delivery.
Further scans may be necessary if abnormalities are suspected. The most common reason for having more scans in the later part of pregnancy is fetal growth retardation. The total number of scans will vary depending on whether a previous scan has detected certain abnormalities that require follow-up assessment.