This is a guide to ultrasound scans and their significance for early pregnancy.
Ultrasound scans send high frequency sound waves through the uterus to allow us to assess pregnancy and fetal wellbeing. These sound waves bounce off the baby and the echoes are turned into images. Hard tissue such as bone give a strong echo and appear white in the image. Softer tissues are varying shades of grey and fluid spaces such as the amniotic fluid around the baby will appear black.
This newsletter will explain all three early pregnancy scans including the Dating Scan, the First Trimester Screening and the Fetal Anatomy/Morphology Scan.
The purpose of the Dating Scan is:
The dating scan is usually attended between 6 and 12 weeks gestation. It usually starts as an ultrasound through the abdomen (called a transabdominal ultrasound). You may need to have a full bladder for this part of the scan as it can help make the ultrasound images clearer. Ultrasound gel is applied to the abdomen to help improve contact between the ultrasound probe and the skin. The abdominal probe is gently moved around the abdomen to obtain ultrasound images.
Commonly in very early pregnancy, even clearer images of the pregnancy can be obtained using an ultrasound probe placed in the vagina (called a transvaginal ultrasound). The transvaginal ultrasound examination doesn’t require a full bladder and uses a probe that is only approx. 2cm in diameter. The probe is covered with a clean, protective disposable sheath before it is gently inserted into the vagina to obtain pictures of the pelvic organs and the developing pregnancy. These images are often clearer as they are closer to all of the structures. If the ultrasound scan date differs from your last menstrual period date, the scan will be used as the most accurate estimate of weeks pregnant and your due date. A dating scan will be carried out at your first consultation visit with Dr. Morris.
Combined First Trimester Screening is carried out between 11 – 14 weeks of pregnancy. This test is a three- part screening process which includes an assessment of:
Structural Anatomy Scan
Combined First Trimester Screening assesses your baby’s risk for chromosomal abnormalities particularly Down Syndrome (Trisomy 21) and two other more common chromosome abnormalities; Edwards Syndrome (Trisomy 18) and Patau’s Syndrome (Trisomy 13). It is possible that some non-chromosomal abnormalities such as neural tube defects, heart and structural abnormalities can also be detected at this stage of pregnancy.
The nuchal translucency is the name for the normal fluid space behind the neck of the unborn baby. During the structural scan, the thickness of this fluid space can be measured precisely and this is known as the NT measurement. The amount of fluid in the neck tissues is known to increase with some chromosomal abnormalities (such as Trisomy 13, 18 & 21) and also some structural conditions, particularly heart abnormalities. An increase in the NT measurement or absence of a nasal bone, or the presence of other anatomical markers does not always mean a baby has a problem, but they do increase the possibility, and this would be discussed with you.
Serum Protein Blood Test
This maternal blood test in the Combined First Trimester Screening incorporates two serum proteins (or markers)
The levels of these naturally occurring proteins are known to change with certain conditions including chromosomal abnormalities. At present this blood test is not used alone to calculate risk, and must be combined with the structural anatomy ultrasound. Low levels of PAPP-A and Placental Growth Factor (PIGF) may also predict issues with the placental function resulting in later growth restriction of the baby, pre-eclampsia and preterm birth. This information will be taken into account in deciding on the frequency of your antenatal visits and ultrasound surveillance. Pregnancy will then be monitored accordingly. This blood test is best taken between 10 and 13 weeks gestation.
A risk rating for chromosomal abnormality is then calculated using some main variables:
The report then gives a risk rating calculated for Trisomy’s 13, 18 & 21. A low risk result does not mean ‘no’ risk. However, a low risk result is reassuring that the baby is healthy.
A high risk result does not always mean the baby definitely has a chromosomal problem. However, the risk is increased and further definitive testing would be discussed. The ultrasound scan alone can detect approx. 70-80% of babies with Down Syndrome.
However, detection is improved by combining the ultrasound features and serum protein markers, with approx. 90% of babies with Down Syndrome detected by Combined First Trimester Screen.
Pre-eclampsia Risk Rating
Pre-eclampsia is a condition unique to pregnancy that is associated with the development of high blood pressure after 20 weeks gestation. Pre-eclampsia can affect the health of a mother, placental function and the growth and wellbeing of the baby in utero. If it occurs early in the second half of the pregnancy (before 34 weeks), it may also result in premature delivery. As part of the First Trimester Screen, a risk factor rating will be determined for a mother’s likelihood of developing the early- onset type of pre-eclampsia.
Assessment will be made of the following:
Preventative medication is offered to women whose risk comes back as higher than 1%.
This scan is recommended between 18 and 19 weeks of pregnancy. It is a combined transabdominal and transvaginal scan and is an examination for fetal abnormalities prior to 20 weeks gestation.
For clearer structural images a full bladder is recommended for the transvaginal part of the assessment.
What This Scan Can Show Us:
Ultrasound uses sound waves, not radiation as x-rays do. Many long term studies have been undertaken on the safety of ultrasound in pregnancy and post birth. None of the properly conducted studies have described a significant adverse effect on either mother or baby and childhood follow-up studies have shown that ultrasound scanning in utero does not adversely affect long term growth or cause developmental delays. All medical authorities and regulatory bodies are satisfied that at the level of the ultrasound power used in assessment of pregnancy, no adverse effect on either mother or baby is expected.
For an explanation of the Non Invasive Pregnancy test (NIPT), please read our blog by Dr. Andrew McLennan from Sydney Ultrasound for Women. Although not an ultrasound scan, this DNA blood test gives increased accuracy alongside the First Trimester Screening for chromosomal abnormalities. Please find the link to our blog here.
By Penny Stanbury, with special thanks to Dr. Greg Kesby for his time and expertise.