Early Pregnancy Scans

Posted on December 14, 2011

An ultrasound scan early in a pregnancy, most likely between six and eight weeks after ovulation, is often considered a standard part of the typical schedule of ultrasound examinations that form a crucial part of prenatal care. A scan at this stage can determine a number of things, despite the foetus still being in a very early stage of development.

An obstetrician uses an ultrasound scan as early as between four or five weeks to confirm gestation. This is done by looking for the chorionic sac – a spherical intrauterine structure containing the young embryonic tissue, appearing on the scan as a dark hypoechoic circle or oval surrounded by white ring, the different colours indicating different tissue types. After five or so weeks, the yolk sac can also be observed, a feature which reliably confirms a true chorionic sac. Detecting this is often a reassuring confirmation of pregnancy for patient. At these early stages, the correlation between the size of the dark area of the chorionic sac enables an effective estimation of the age of the gestation, especially in the period between five and seven weeks before the embryo becomes visible, generally with an error margin of as little as five days either way. However, it becomes a less effective indicator as soon as the embryo becomes visible on the scan because the correlation becomes less reliable, which is usually after about six or seven weeks.

From this point, the crown-rump length of the embryo becomes the critical parameter by which the age of the gestation is estimated, with a smaller margin for error. It will also be around this point when embryonic cardiac activity becomes observable. Determining the age of the gestation is an important part of the obstetric examination for a number of reasons, such as determining due date and perhaps the paternity of the foetus, and scheduling further examinations.

Of course, an ultrasound scan at five weeks can also be crucial in screening for gestational or developmental abnormalities. The positioning of the chorionic sac is the best way to diagnose an ectopic pregnancies, which is when the embryonic tissue implants somewhere other than the uterus, most likely in the fallopian tube. This will almost certainly mean the gestation isn’t viable, and can be extremely dangerous to the mother, so must be checked for and diagnosed early if present. Also, due to the early detection of the heartbeat of the developing embryo, an obstetrician is able to check for healthy cardiac activity. An ultrasound scan at eight weeks showing normal foetal development and size is a very strong indicator of a successful live delivery.

This entry was posted in News, Obstetric

Tom Farrell

Written by Tom Farrell

Tom Farrell works in both the NHS and private sector providing gynaecology, pregnancy ultrasound and maternity care clinics in the Jessop Wing (NHS) and BMI Thornbury & Claremont Hospital (Private sector) in Sheffield.

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