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| The early pregnancy scan (or fetal viability scan) is usually done between 5 and 12 weeks. A trans-vaginal probe is used for this examination. The scan will provide the following information:
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- Detection of fetal heart movement/rate. Seeing the heart beating is reassuring and confirms that the baby is okay.
- Number of fetuses. Determination of chorionicity in twin pregnancy (identical twins) for example can be made early and extra surveillance of these high risk pregnancies can be organized to optimize the outcome.
- Ensure an intra-uterine pregnancy. It confirms that the fetus is well placed in the womb and therefore excludes an out-of-place (or ectopic) pregnancy.
- Detection of structural abnormalities. Gross fetal abnormalities such as anencephaly (fetus without a brain) can be detected early in pregnancy.
- Determines the gestational age of the fetus. This can be made accurately by measuring the crown-rump length (CRL). It will help in determining the expected date of delivery (EDD).
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The fetal viability scan is important for women who experience bleeding in pregnancy, previous miscarriages or fetal abnormalities in pregnancy. It is often used to reassure women that their pregnancy is off to a good start.
The gestational sac can be seen as early as four and a half weeks of pregnancy and a yolk sac at about five weeks. A trans-vaginal probe has to be used to visualize the early pregnancy. This special transducer is introduced gently into the vagina. Clearer images are obtained because the probe is of higher frequency and is nearer to the object of interest in the womb. Furthermore, the examination does not require a full bladder but it is slightly more uncomfortable as compared to the trans-abdominal probe.
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A gestational sac can be seen as
early as 5 weeks with a yolk sac |
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Fetal heart movement confirms
fetal viability |
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Any bleeding in pregnancy is abnormal. Ultrasound scan is used to confirm viability of the fetus in the event of vaginal bleeding in early pregnancy. Fetal heart movements can be seen as early as 6 weeks of gestation, with the trans-vaginal transducer. When this is seen, the outcome for this pregnancy to continue on normally is good; with a probability of more than 95%.
Miscarriage in the form of a "blighted ovum" or "missed abortion" can be diagnosed easily when an empty gestational sac is seen after 7-8 weeks of pregnancy or when the fetal heart movement is absent. Ultrasound is also used to detect the presence of retained products of conception when the bleeding is especially heavy and accompanied by lower abdominal pain. In this situation, the doctor will advise an ERPC (evacuation of products of conception) or dilatation and curettage (D&C). Ultrasound scan can also help to rule out or confirm suspected ectopic pregnancy. This will allow diagnosis of an out-of-place pregnancy, usually located in the fallopian tubes or abdomen instead of within the womb.
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An empty gestational sac,
measuring more than 2cm,
with no fetus indicating
a "missed abortion". |
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An absent fetal cardiac activity
also indicates a missed abortion.
The surrounding vessels are
seen with colour power angio but
there is no cardiac activity
in the chest. |
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The gestational age of the baby can be estimated by measuring the fetal body. This is particularly important in a woman who is uncertain of her last menstrual period (LMP) or has an irregular period. Measurements of the fetus in early pregnancy will help the doctor arrive at a correct dating for this woman and therefore a more accurate estimation of the gestational age. It will allow an accurate estimation of the expected date of delivery (EDD).
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Fetal heart movement video
confirming viability of the fetus |
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Measurements of the fetus that are usually taken include the crown-rump length (CRL), biparietal diameter (BPD) and the femur length (FL). The CRL measurement is made between 7 to 13 weeks of gestation and allows the best estimation of gestational age within 3-5 days. The BPD measurement is made from 13-24 weeks to estimate the gestational age of the baby, after which it becomes inaccurate. The BPD measures the distance between the outermost points of the baby's head. Its accuracy in estimating gestational age is within 7 days. FL measures the baby's thigh bone. It can be used to estimate the gestational age from 13-24 weeks gestation to within 7 days.
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Common measurements taken to estimate gestational age;
CRL (a), BPD (b), FL (c) |
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The crown-rump-length (CRL) offers the best estimate of the gestational
age to within 3 to 5 days. However, it has to be measured between
6-13 weeks. This scan image shows the CRL measurement at
8 weeks gestation. |
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The confirmation of twins and other multiple pregnancies can be easily made by the ultrasound scan in early pregnancy. Ultrasound can also determine the "chorionicity" of twin pregnancies to confirm or exclude monozygotic pregnancies (identical twins). This is important because monozygotic pregnancies are associated with higher risks of fetal abnormalities, growth problems and the twin-to twin transfusion syndrome. This will also allow increased surveillance of these high-risk pregnancies, so that maternal and fetal complications can be anticipated and detected early. Plans can be formulated to optimize care for the mother to improve the outcome for her babies.
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Ultrasound can be used to diagnose multiple gestation;
twin pregnancy (a) and triplet pregnancy (b).
Early ultrasound can also confirm exclude identical twins
using the "lambda" sign, λ or the "twin peak" sign (c), (d). |
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