Early pregancy scan. This scan is best performed transvaginally, although a transabdominal examination can be performed at the patient’s request.
The scan is usually performed for a specific indication such as pain or bleeding in early pregnancy, to confirm multiple pregnancy in patients who have had fertility treatment or to confirm early viability in women who have previously had a miscarriage or an ectopic pregnancy.
Scanning at this gestation is to:
This provides important information about your pregnancy. It is usually performed transabdominally but occasionally a transvaginal scan will be required to provide all the information.
Scanning at this gestation is to:
Confirm the viability of a pregnancy
At this gestation although the majority of pregnancies will be normal, a small number will result in early pregnancy loss
Confirm the gestational age
Accurate dating of a pregnancy is particularly important for women unsure of their menstrual cycle or those with irregular cycles.
Diagnose multiple pregnancy
This may be present in up to 2% of pregnancies at this gestation, and is even more common if ovulation induction has been undertaken. This is the best gestation to determine whether or not twins share a placenta (chorionicity) which has a major impact on the management of the pregnancy.
To screen for chromosomal abnormalities (combined nuchal-biochemistry test)
Although most babies are normal, all mothers have a risk of having a baby with a chromosomal anomaly including Down syndrome. Measurement of the nuchal translucency (i.e fluid behind the neck of the baby), together with knowledge of the mother’s age and measurement of the maternal hormones BhCG and PAPP-A will allow estimation of the mother’s individual risk. Mothers with a high predicted risk (usually 1:300 or greater) may then decide if they wish to have diagnostic testing (amniocentesis or chorion villus sampling).
Diagnose major structural abnormalities
A number of major fetal abnormalities including severe forms of spina bifida and diaphragmatic hernia may be diagnosed at this gestation. However, we recommend a detailed fetal anatomy scan at 20-23 weeks for optimal detection of fetal anomalies.
This scan is performed transabdominally. It will confirm that the vast majority of pregnancies are developing normally. However, about 1% of all fetuses will have a serious congenital abnormality and ultrasound examination at this stage will detect about 70% of these. In addition, screening tests for the later development of high blood pressure (pre-eclampsia), placental insufficiency, poor fetal growth or intrauterine growth retardation (IUGR) and premature labour can be performed.
Detailed inspection of every organ and part of the fetal anatomy is performed, including the fetal brain, spine, heart, lungs, kidneys, bowel and bladder, limbs, fingers and toes.
Pre-eclampsia and IUGR
These conditions are caused by poor inversion of the placenta into the wall of the uterus. Prediction of the possible development of these problems can be made by performing Doppler examination of blood flow in the uterine arteries.
About 50% of cases of spontaneous delivery before 34 weeks are associated with shortening of the cervix. Ultrasound assessment of the cervix can be performed at this assessment, usually transvaginally.
Although rare, we advocate screening for this abnormality. If your pregnancy is at increased risk or sub-optimal, views are obtained on ultrasound for technical reasons, we would recommend detailed cardiac scanning by a fetal cardiologist.
The assessment of fetal well-being from 24 weeks involves the study of several different parameters.
Fetal growth is determined by measurement of the head, abdomen and femur length. It is then possible to estimate fetal weight which may be compared with the expected growth for that gestation.
Measurement of the liquor volume is a good indicator of pregnancy complications including diabetes and placental insufficiency.
The observation of fetal movements and fetal breathing movements are also made and are indicators of fetal wellbeing.
Colour flow Doppler to assess fetal blood flow and maternal blood flow in the uterine arteries is a sensitive marker of the health of the pregnancy. In some cases, detailed assessment of the fetal blood vessels may be recommended by your consultant.
The position of the placenta in relation to the cervix is checked to exclude a placenta praevia. If the placenta was found to be “low lying” at your 20 week scan, your consultant will usually advise such a scan.
Indications for serial scans::
The growth scan at 28-34 weeks is often the best time for obtaining 3D and 4D views of the baby.
Detailed examination of the fetal heart is performed by a consultant fetal cardiologist.
Some groups of pregnant women are recognised to be at increased risk for heart problems in their fetus.
These groups include:
Parents may value fetal echocardiography to provide reassurance of normality even in the absence of other risk factors.
Scans are usually performed at 18-23 weeks gestation, but can be undertaken from as early as 14 weeks gestation if required.
If an abnormality is found, parents will receive a detailed explanation and prognosis of the cardiac condition and further investigation and management will be planned as appropriate.
These are diagnostic tests.
The commonest indication for these tests is to check the baby’s chromosomes, but may be used to diagnose inherited conditions such as cystic fibrosis, Tay-Sachs, sickle cell or haemophilia.
It does this by analysing fetal cell-free DNA circulating in the maternal blood. Because the test is non-invasive there is no risk for miscarriage or other complications associated with invasive testing procedures.
Chorion Villus Sampling (11-20 weeks)
A thin needle is passed through the abdomen to sample the placenta (afterbirth). Both baby and placenta develop from the same cells, therefore the chromosomes present in the placenta can be used to check the chromosomes in the baby.
CVS is not performed before 11 weeks. The procedure takes less than two minutes and is performed under scan guidance.
A fine needle is passed through the abdomen and amniotic fluid around the baby is taken and tested for abnormalities.
Amniocentesis is not performed before 16 weeks. The procedure is very quick and takes less than two minutes and is performed under scan guidance.
A result to exclude Downs Syndrome is available in three days.
The risk of miscarriage associated with these tests is 1%. In approximately 1:1000 cases the test will need to be repeated because the cells will not grow in the laboratory or the results are inconclusive.