There is no definitive rule as to the number of scans a woman should have during her pregnancy. Some physicians will order an ultrasound when an abnormality is suspected on clinical grounds, while others advocate screening ultrasounds. You should consult your health care provider to determine the most appropriate scanning schedule for you.

Scans may be performed in the first trimester to:

• Confirm a normal intra-uterine pregnancy
• Assess fetal age
• Exclude abnormalities such as ectopic pregnancies or potential for miscarriage
• Assess fetal heart activity
• Determine the presence of multiple pregnancies
• Identify abnormalities of the placenta, uterus, and other pelvic structures

Scans may also be obtained in the second and third trimesters to:

• Assess fetal age, growth, position and sometimes gender
• Identify congenital malformations
• Exclude multiple pregnancies
• Evaluate the placenta, amniotic fluid, and remaining structures of the pelvis

Some centers are now performing a scan at around 13-14 weeks of pregnancy to screen for findings that may represent a risk for Down Syndrome (a type of chromosomal defect which causes mental retardation) or other developmental abnormalities, in the fetus.

The total number of scans will vary depending on whether a previous scan or blood tests have detected abnormalities that require follow-up assessment.

Abnormal ultrasound results may be due to some of the following conditions:

• Ectopic pregnancy
• Multiple pregnancies
• Fetal death
• Abnormalities of fetal position
• Congenital malformations
• Amniotic fluid problems, including oligohydramnios (not enough fluid) and polyhydramnios (too much fluid)
• Placental abnormalities, including placenta previa and placental abruption
• Intrauterine growth retardation
• Tumors of pregnancy, including gestational trophoblastic disease
• Additional abnormalities of the ovaries, uterus, and remaining pelvic structures