Pregnancy ultrasound

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Alternative names
Pregnancy sonogram; Obstetric ultrasonography; Obstetric sonogram; Ultrasound - pregnancy

Definition
Pregnancy ultrasound is a method of imaging the fetus and the female pelvic organs during pregnancy. The ultrasound machine sends out high-frequency sound waves which bounce off body structures to create a picture.

How the test is performed
You will be lying down for the procedure. A clear, water-based conducting gel is applied to the skin over the area being examined to help with the transmission of the sound waves. The ultrasound transducer (a hand-held probe) is then moved over the abdomen and pelvis. This is the conventional transabdominal technique.

How to prepare for the test
Since a full bladder is necessary for improved imaging, you may be asked to drink 2 to 3 glasses of liquid 1 hour before the test. You should not urinate before the examination.

How the test will feel
There may be some discomfort from pressure on the full bladder. The conducting gel may feel slightly cold and wet. You will not feel the ultrasound waves.

Why the test is performed

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.

Normal Values
The fetus and associated pelvic structures are normal in appearance and appropriate for the gestational age.

What abnormal results mean

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

What the risks are
There is no documented biologic effect on patients and their fetuses with the use of current ultrasound techniques. No ionizing radiation is involved.

Special considerations
Transvaginal ultrasound scanning may be performed with the probe placed in the vagina of the patient. This technique often complements conventional ultrasound techniques by providing better anatomic detail. Consult your health care provider to determine which technique is most appropriate for you.

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