Boulcott Hospital

First Trimester Ultrasound Scan – Benefits Outweigh the Costs? by Mr Ganesh A. Ganeshalingham

Diagnostic ultrasound in the first trimester deals with pregnancy dating, the recognition of early pregnancy failure/loss (embryonic demise, spontaneous abortion, ectopic pregnancy, placental abnormalities), multiple pregnancy and with the recognition of normal anatomy and/or the detection of anomalies in very early viable pregnancies.

The application of ultrasound in pregnancy may be broadly classified as either elective or reactive. Elective or planned use implies scanning to detect potential problems in otherwise seemingly uncomplicated pregnancy (screening), whereas reactive use is the application of ultrasound to help in the management of a clinical problem (diagnosis).

A: ELECTIVE USE OF ULTRASOUND (screening)

Pregnancy dating:

The use of clinical factors, history and examination, lead to a much wider error margin in pregnancy dating than when ultrasound is used in the first half of the pregnancy. Ultrasound during the first trimester is the most accurate method to establish dating. During this period, gestational age is by far the strongest variable affecting foetal size and the crown-rump measurement is accurate within five days. In the second trimester, dating by bi-parietal diameter has an error deviation of ten days.

Multiple Pregnancy:

Twins account for about 1% of all pregnancies with two-thirds being dizygotic and one-third monozygotic (identical).

• Dizygotic pregnancies are dichorionic.
• Monozygotic pregnancies may be dichorionic
• or monochorionic (share same placenta).

The perinatal mortality rate in twin pregnancy is around six times higher than in singletons. This increased mortality is about 3-4 times higher in monochorionic than dichorionic twin pregnancies, regardless of zygosity. Determination of chorionicity is important in the management of twins. The best way to determine chorionicity is by ultrasound examination at 6-9 weeks of gestation when in dichorionic twins there is a thick septum between the chorionic sacs. This septum becomes progressively thinner after the first trimester and is difficult to identify later in the pregnancy.

Chromosomal aneuploidy:

Nuchal translucency Assessment (NT):
Foetal nuchal translucency thickness (NT) is the accumulation of fluid behind the foetal neck in abnormal foetuses. NT normally increases with gestation (CRL). There is an association between the ultrasonic finding of increased nuchal translucency thickness in the first trimester and subsequent diagnosis of chromosomal defects. A combination of maternal age and increased nuchal translucency relative to the mean for gestation will detect approximately 80% of foetuses with Down’s syndrome and about 70% of foetuses with other chromosomal abnormalities. The minimum foetal CRL should be 45 mm and the maximum 84 mm. The optimal gestational age for measurement of NT is 11-13 + 6 weeks. The success rate for taking a measurement at this gestation is 98-100%, falling to 90% at 14 weeks; from 14 weeks onwards, the foetal position (vertical) makes it more difficult to obtain measurements and NT usually resolves by 14 weeks gestation. Screening for chromosomal defects in the first rather than the second trimester has the advantage of earlier prenatal diagnosis and consequently less traumatic termination of pregnancy for couples who choose this option.

Abnormal NT with normal karyotype:
Where the NT measurement is increased by the karyotype is normal, a variety of structural abnormalities have been reported, therefore these foetuses constitute a high risk group that requires careful second trimester morphologic and echocardiographic assessment.

Counselling women in NT assessment:
The choice of appropriate screening and diagnostic tests for aneuploidy and the resultant decision making pathway chosen, is largely influenced by the needs and the beliefs of the individual patient. The utilisation of the available tests and the significance of the results will vary considerably depending on those individual needs and beliefs. All the tests have advantages and disadvantages (please refer to Table 1 for further information), and are based on differing techniques for the assessment of the pregnancy.

Counselling guidelines:

When counselling patients regarding the assessment of aneuploidy, it is essential that several key issues are addressed and the patient understands these issues. In order to ensure counselling is thorough and comprehensible the following points are suggested for inclusion in any discussion with patients:

• An explanation of the difference between screening and diagnostic tests.

• An explanation that low risk does not equal no risk. It is essential that patients understand that low risk or “good” NT result does not exclude Trisomy 21 or other anomalies.

• A discussion of the patient’s perceived “priority”. It is important to realise that this may change as results become known.

• A discussion of the purpose, accuracy, disadvantages, advantages, timing and costs of the tests

• The results of tests should be discussed in the context of possible further testing or decision.

• A discussion of the possible implications of the results.

• The opportunity for the patient to ask questions or for clarification of any issues.

• If the result is “abnormal”, that is increased risk, the following points need to be emphasised.

• An increased NT is not in itself an abnormality; it simply provides risk assessment.

• An increased NT is not only seen in aneuploidy; there is a wide range of other conditions that have been reported in association with increased NT.

• The first trimester ultrasound examination can identify about 85% of chromosomal and 70% of structural anomalies.

Detection of foetal structural abnormality:

The early pregnancy scan was initially introduced with the primary intention of measuring the foetal CRL to achieve accurate pregnancy dating. During the last decade, however, improvement in the resolution of ultrasound machines has made it possible to describe the normal anatomy of the foetus and to diagnose or suspect the presence of a wide range of foetal defects in the first trimester of pregnancy.

B: REACTIVE USES: (ASSIST IN DIAGNOSIS AND MANAGEMENT)

The primary purpose of ultrasound is to determine the site, number and viability of foetuses. Distinguishing between viable and non viable intrauterine pregnancies may be difficult. If there is any doubt about the viability of an intrauterine pregnancy, the patient should be reviewed in no less than seven days.

Assessment of early pregnancy failure/loss:

Live intrauterine pregnancy:

Definition: A normally sited gestation sac with clearly identified cardiac activity in an embryo.

Diagnosis: A probable normal pregnancy. 90% of women will not miscarry. Perinatal outcome for the first trimester bleeding is little different from the general obstetric population.

Foetal pole, no cardiac activity (Very early normal pregnancy/embryonic demise):

Definition: This is a normally sited gestation sac with a clearly identified embryo or foetus but no cardiac activity.

Diagnosis: Some of these pregnancies will be viable but others will represent early foetal death (embryonic demise). The ultrasound criterion used is the crown-rump length (CRL):

Empty gestation sac:

Definition: This is a normally sited true gestation sac with no embryo. It is important to remember that the appearance of an intrauterine sac (pseudosac) will occasionally be seen with ectopic pregnancy.

Diagnosis: Some of these pregnancies will be viable, in that it may be a very early pregnancy, but others will represent an anembryonic pregnancy (blighted ovum).

Retained product of conception (incomplete miscarriage):

Definition: The uterus contains tissue of mixed echogenicity with no gestation sac.

Diagnosis: The tissue generally represents retained products of conception but a retained blood clot may give a similar appearance.

Empty uterus (with positive pregnancy test):

Definition: The uterus contains no tissue or decidual reaction only.

Diagnosis: There are three possible explanations

1. Very early pregnancy.
2. Complete miscarriage or
3. Ectopic pregnancy.

A transvaginal scan (TVS) may provide more information. On TVS, very early intra-uterine pregnancies are often identifiable by 5-6 weeks of gestation (presence of yolk sac and/or embryo) and ectopic pregnancies are more easily seen.

Equally, it must not be forgotten that the absence of any direct evidence for ectopic pregnancy at TVS does not exclude the diagnosis.

Distinguishing between the three possibilities is aided by a detailed history, examination and measurement of serum beta hCG levels. The aim is to avoid unnecessary laparoscopy with a very early intrauterine pregnancy while averting the risk of maternal collapse from a neglected ectopic gestation.

Serum level of beta hCG assists by ruling out the possibility of a normal intrauterine gestation; once the concentration is above 1000 iu/l an intrauterine pregnancy is visible at transvaginal scan.

If the beta hCG <1000 iu/l:

Ectopic pregnancy, intrauterine pregnancy and complete abortion all possible. Ectopic pregnancy is unlikely to rupture in the next 48 hours.

If no risk factors and no evidence of peritonism, the patient can be reviewed every 48 hours with a repeat transvaginal scan and serum beta hCH levels. If the beta hCG falls or has not doubled within 48 hours, then a normal pregnancy has been excluded.

If the beta hCG >1000 iu/l:

Ectopic pregnancy or complete abortion possible; consider laparoscopy or review in 48 hours.

If the beta hCG is greater than 1000 iu/l and there is no intrauterine pregnancy, the management depends on the likelihood of ectopic pregnancy. In some cases it will seem highly likely that the woman has had a complete miscarriage. If necessary, this can be confirmed by a beta hCG fall to 20% of the original level by 48 hours. In other cases there is clear evidence of ectopic pregnancy on clinical or ultrasound examination and laparoscopy is indicated. Difficulty arises when there is substantial doubt about the diagnosis. If the serum beta hCG level has followed a decay curve to 20% of the original value, complete miscarriage is diagnosed. If the level has risen or fallen more slowly, ectopic pregnancy is likely and laparoscopy should be considered.

Suspected trophoblastic disease (hydatidiform mole):

Definition: The uterus contains tissue with ultrasonographic features of hydatidiform mole, either with (partial mole) or without (complete mole) a gestation sac. Very rarely a complete mole may co-exist with a normal pregnancy.

Diagnosis: The ultrasound appearances of a large complete hydatidiform mole are virtually diagnostic but those of a partial mole in early pregnancy are often more subtle.

Extra chorionic (subchorionic) Haematoma:
Subchorionic accumulation of blood is not uncommon in the first trimester and may be associated with vaginal bleeding. The prognosis appears to depend on the volume of the haematoma the smaller, the better.

Conclusion:

First trimester ultrasound has many advantages, including accurate dating, detection of early pregnancy failure/loss, and identification and characterisation of twin pregnancies. First trimester NT screening can detect approximately 80% of foetuses with chromosomal abnormalities. Furthermore, about 70% of structural abnormalities are detected in the first trimester. To be most cost effective, first trimester scan should be performed between 11-14 weeks (unless there is indication earlier because of pregnancy complication); this would be an ideal time for accurate dating with CRL and is the optimal time for NT assessment when the CRL is 45-84 mm. NT usually resolves by 14 weeks.

Tabel 1: Tests for the Assessment of Aneuploidy

CHORIONIC VILLOUS SAMPLING (CVS):

Test type: Diagnostic

Procedure: Direct sampling of placental tissue at 11-13 weeks.

Advantages: Early and rapid results.

Accuracy 97%.

Disadvantages: Miscarriage rate about 1-2 %.

More uncomfortable than amniocentesis. Technically can be difficult. Higher method failure than amniocentesis.

NUCHAL TRANSLUCENCY THICKNESS:

Test type: Screening

Procedure: Ultrasound scan at 11-13.6 weeks gestation to measure CRL and NT and to assess for major structural anomalies.

Advantages: Relatively quick, painless procedure. Allows early assessment of dating and viability. Accuracy for detection of foetuses at high risk of chromosomal defects about 80%.

Disadvantages: Risk assessment only, not a definitive test. Technical precision of measurements and access to computer programme.

MATERNAL AGE ASSESSMENT:

Test type: Screening

Procedure: Notation of maternal age and comparison with arbitrary “high-risk” cut off age.

Advantages: Quick, simple, completely non-invasive.

Disadvantages: Accuracy about 30% (age 37). No other information obtained regarding pregnancy status.

MATERNAL SERUM TESTING:

Test type: Screenings

Procedure: Maternal venous blood sample obtained and analysed at 15-18 weeks gestation.

Advantages: Non-invasive assessment of risk for chromosomal and neural tube defects. Relatively inexpensive and painless.

Disadvantages: Erroneous results if dates incorrect. Risk assessment only, not definitive answer. Accuracy about 60%.

AMNIOCENTESIS:

Test type: Diagnostic

Procedure: Direct sampling of the amniotic fluid from 15 weeks gestation.

Advantages: Most accurate assessment of foetal chromosomal status (99%). Relatively quick and less uncomfortable than CVS.

Disadvantages: Miscarriage rate about 1%. Delay in full results (14 days or more). This can be lessened with FISH analysis. Second trimester test.

18-20 WEEK MORPHOLOGY SCAN:

Test type: Screening

Procedure: Detailed ultrasound examination of the foetus and environment to assess for structural defects and variations (markers) at 18-20 weeks gestation.

Advantages: Relatively quick, usually positive bonding experience for the mother. A significant amount of detailed information regarding the progress of the pregnancy can obtained.
Disadvantages: Highly operator dependent. Good quality equipment and training required for accurate results. Accuracy for detection of foetuses at high risk for chromosomal defects: 50% for trisomy 21, 80% for trisomy 13 and 18. Trisomy 21 may not be detectable if no markers present.