Use of Ultrasound Scanning in Pregnancy

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Use of Ultrasound Scanning in Pregnancy

Use of Ultrasound Scanning in Pregnancy: Ultrasound scan is a safe, non-invasive,
accurate and cost-effective investigation in pregnancy for routine assessment and in the investigation of certain abnormalities.


Routine ultrasound timing


in the first trimester.
at 18—22 weeks (Second Trimester ).
in the third trimester.

Ultrasound Scanning First Trimester in Pregnancy


The first trimester scan is performed to:

  • confirm pregnancy foetal viability.
  • accurately estimate the gestational age by measuring the crown rump length. The accuracy of prediction is 5 days.
  • exclude pelvic pathology such as fibroids, ovarian tumours and uterine abnormalities.
  • diagnose multiple pregnancy and to determine the chorionicity (at 11—13 weeks). In dichorionic twins the two amniotic cavities are separated by a thick three layered membrane, consisting of the fused amnion in the middle and chorion on either side and appears as a lambda sign on ultrasound scanning. In monochorionic twins the dividing membrane is thin and consists of a single layer of amnion and appears as a T-sign on ultrasound scanning.
  • measure the nuchal translucency which is a marker for Down’s syndrome and other trisomies (at 11—13 weeks).
  • investigate bleeding in the first trimester and to diagnose missed abortion, hydatidiform mole and ectopic pregnancy. A missed
    abortion or a blighted ovum is diagnosed by the presence of a deformed gestational sac and absence of a foetal pole or heart
    beat. However, a repeat scan may be necessary to confirm the diagnosis if there is a doubt regarding the dates. A hydatidiform
    mole is diagnosed by demonstrating the typical snow storm appearance.
  • perform chorionic villous sampling.

Ultrasound Scanning Second Trimester in Pregnancy


The second trimester scan is routinely done at 18—22 weeks to enable accurate diagnosis of foetal abnormalities and estimation of the gestational age.

  • To confirm dates by measuring the BPD, femur length and the head circumference.The accuracy of prediction is 7 days.
  • To detect foetal structural abnormalities.
  • To localize the placenta. A small number of women who have a low lying placenta at 20 weeks will develop a placenta praevia.Placenta praevia is confirmed in the third trimester.
  • To measure cervical length in patients at risk of preterm labour. A transvaginal scan is required for this purpose.
  • To estimate the amniotic fluid volume.
  • To diagnose multiple pregnancy.
  • To perform amniocentesis and cordocentesis.
  • Serial ultrasound scanning is done in twin pregnancies for early detection
  • To perform uterine artery Doppler studies. A high resistance index and/ or bilateral notching at 20—24 weeks, are of predictive value for the occurrence of pre-eclampsia, intrauterine growth restriction and placental abruption.

Ultrasound Scanning Third Trimester in Pregnancy

  • To assess foetal growth and the estimated foetal weight
  • To confirm foetal presentation.
  • To confirm placenta praevia.
  • To estimate the amniotic fluid volume.
  • To perform amniocentesis and cordocentesis. The middle cerebral artery peak systolic velocity is assessed in the surveillance of affected foetuses in Rhesus iso-immunisation.
  • To assess foetal well-being

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