25.1
Introduction
The prevalence of obesity is on the rise and is a major health concern in pregnancy. It is associated with comorbidities posing risks to the health of both the mother and her unborn foetus. Planned optimum antenatal care commencing from the early weeks of pregnancy can help to identify and mitigate many of these risks, such that adverse outcomes can be avoided.
In this chapter, we will describe ultrasound scanning in early pregnancy and foetal abnormality screening in obese women.
25.2
Ultrasound scanning in early pregnancy
25.2.1
Role of early pregnancy scan in obese pregnant women
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An early pregnancy scan in obese pregnant women can be quite informative for planning antenatal care and is also reassuring because of uncertainty about the last menstrual period.
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Obesity is often associated with increased rates of anovulatory cycles with subsequent irregular cycles and subfertility.
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The increased need for assisted conception leads to an increase in twin and multiple pregnancies.
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A significant proportion of obese pregnant women also have Polycystic Ovarian Syndrome, which has been associated with early pregnancy losses.
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Obesity is also a risk factor for caesarean section in their previous pregnancy.
Approach to an early pregnancy scan
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Assess first by transabdominal examination with full bladder—use linear transducer, anticipate poor visualisation.
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Transvaginal examination with high-frequency transducer and with an empty bladder:
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Confirm intrauterine pregnancy and rule out ectopic pregnancy.
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Establish viability of pregnancy and document foetal cardiac activity.
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Measure gestational sac, Crown-rump length (whichever or both if applicable) and yolk sac to correlate with period of gestation.
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Note the number of foetuses by scanning in both transverse and longitudinal axis of the uterus.
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Note evidence of any haematoma or bleeding in or around gestational sac.
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Make a note of the previous caesarean scar (if applicable) and its relation to the gestational sac.
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Examine both adnexa, pouch of Douglas and uterus to note any pathology like fibroids, ovarian cysts, hyperstimulated ovaries, or Mullerian malformations.
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Infrequently, major foetal malformations like body stalk anomaly, anencephaly, and conjoined twins can be diagnosed by transvaginal early pregnancy scan.
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25.2.2
First Trimester screening (nuchal translucency scan and dual marker)
According to NICE guidelines, all pregnant women should be offered combined first trimester screening to estimate their risks for common aneuploidies. Apart from confirming viability, gestational age, and number of foetuses, an important component of the first trimester ultrasound is to carry out the nuchal translucency (NT) measurement in order to screen for chromosomal abnormalities, and it may even detect major foetal structural anomalies.
Dual marker consists of maternal serum beta HCG and PAPP-A measurements using specific assays. Both NT and dual marker measurements are used to derive an individualised numerical risk for common aneuploidies (Trisomy 21, 18, and 13) against the woman’s background risk, which is determined primarily by her age.
The early foetal anatomic assessment may prove most satisfactory when carried out by transvaginal ultrasound, rather than transabdominal ultrasound in the obese population.
Transvaginal scan is an important tool to detect major congenital abnormalities like anencephaly, holoprosencephaly, anterior abdominal wall defects, etc. Numerous studies have shown that visualisation and hence detection of major structural defects, including cardiac abnormalities can be as high as 90%with transvaginal examination. The importance and utility of a transvaginal sonographic assessment in obese women cannot be overemphasised.
Considerations specific to screening in pregnant women with raised BMI
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NT measurements in obese women can be difficult and often require longer scanning time and repeated measurements. Additionally, imaging of foetal nasal bone may be suboptimal due to maternal habitus.
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The levels of biochemical markers like b-HCG and PAPP-A are variably influenced by maternal weight and the values of these analytes are lower in obese women presumably due to increased plasma volume and dilutional effects. Inaccurate information can lead to significant alterations in the estimated risk and result in screening failure. Therefore, it is important to provide laboratories with correct information about maternal weight for accurate risk estimation.
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Noninvasive prenatal testing (NIPT) by cell-free foetal DNA requires a foetal fraction above 3%–4% to ensure the reliability of results. Increased maternal BMI is associated with a decrease in the foetal fraction and a higher rate of “no call” NIPT results, hence the need for repeated samples or further invasive testing with their inherent risks. Women should be counselled about known limitations of NIPT in the setting of maternal obesity.
Caution regarding use of Dopplers in the first trimester: Embryonic period (from conception till 9+6 weeks) is characterised by rapid cell division and development of the embryonic organs.
According to updated ISUOG safety guidance 2021, spectral Doppler, colour flow imaging, power imaging, and other Doppler ultrasound modalities should not be used routinely in the embryonic period. If use of Doppler is clinically indicated, then the exposure time should be kept to a minimum.
In the foetal period (CRL>45 mm), Doppler ultrasound modalities may be used routinely for certain clinical indications, such as screening for trisomy and cardiac anomalies. When performing Doppler ultrasound, the displayed thermal index should be ≤1.0 and exposure time should be kept as short as possible (usually no longer than 5–10 minutes).
25.2.3
Fetal abnormality screening in obese women
Foetuses of obese mothers are more likely to have congenital malformations compared to foetuses of mothers with normal prepregnancy weight, as depicted in Fig. 25.1 .
