Abstract and Introduction
First-trimester prediction of intrauterine growth restriction (IUGR), preeclampsia (PE), birthweight, aneuploidy, miscarriage, complications in multiple pregnancies and homozygous α0-thalassemia is a challenging and emerging field. Placenta volumes (PV) and embryo volume/fetal volume ratios are correlated with crown–rump length (CRL) or gestational age. Measurement of PV or placental quotient (PV/CRL ratio) is an early method to identify impaired trophoblast invasion and predict subsequent development of IUGR or PE. In early-onset IUGR caused by triploidy, or trisomy 13 or 18, a larger deficit in fetal volume than CRL is observed. Fetal and placental volume measurements may be applied to predict other conditions such as aneuploidy, miscarriage or stillbirth. Standardization of the 3D volumetric methodology is needed to improve reproducibility of measurement. Further studies are required to determine the use of first-trimester volumetry alone or in combination with Doppler ultrasound and other parameters to achieve the best predictive models for IUGR and PE.
The development of 3D ultrasound has enabled us to measure the volumes of the embryo (EV), fetus (FV),[1,2] placenta (PV),[3,4] gestational sac (GSV)[1,5] and yolk sac (YSV)[1,6] reliably in the first trimester. A strong correlation between EV/FV ratio,[2,8] PV, GSV and crown–rump length (CRL) or gestational age (GA) has been shown.[4,8–10] An increase in FV or PV over gestation was much greater than CRL.[4,9]
Several studies have been performed to investigate the use of first-trimester volumetry in the prediction of intrauterine growth restriction (IUGR; failure of the fetus to achieve its growth potential) and preeclampsia (PE; proteinuric hypertension in pregnancy),[11–14] birthweight,[15,16] aneuploidy,[17–21] miscarriage,[22–24] complications in multiple pregnancies,[25,26] homozygous α0-thalassemia and other adverse outcomes. The pathophysiology of both IUGR and PE includes impairment of trophoblastic invasion of the maternal spiral arteries and their conversion from narrow muscular vessels to wide nonmuscular channels. The traditional prediction methods include maternal history, 2D ultrasonography and biochemical markers.[28–32] The use of first-trimester uterine artery Doppler (UAD) and placental biochemical markers (including pregnancy-associated plasma protein A, PGF, soluble fms-like tyrosine kinase 1, P-selectin and neutrophil gelatinase-associated lipocalin) can detect 77–88.9% of PE at a 10% false-positive rate,[28–30] but present biochemical markers or UAD alone are not sufficiently accurate.[3–35] The sensitivity of UAD in predicting small-for-gestational-age (SGA)/IUGR was low, ranging from 14 to 37%,[32,34] and for late PE was not high, at approximately 57.0%.
Measurement of the gestational sac diameter and CRL has been used to determine GA and for the evaluation of miscarriage. However, measuring CRL can be less reliable before 7 and after 10 weeks' gestation because of amorphous embryonic contour and fetal movements, respectively. Sonographically thick placenta at the second or third trimester is associated with SGA or large-for-gestational-age infants at term. Increased placental thickness at the first trimester is a sonographic sign of homozygous α0-thalassemia, but its sensitivity is 72%.
In general, the use of 3D ultrasound to measure volumes of regularly or irregularly shaped objects is more accurate than 2D ultrasound, and is sufficiently accurate and reliable for clinical use.[38,39] Volume calculations are commonly performed using the multiplanar method (which is more time consuming) or a rotational method with virtual organ computer-aided analysis (VOCAL™) software (GE Medical Systems, Zipf, Austria).[2,4,5,7,33] Newer volumetric tools include a semiautomated virtual reality (VR) system, ISpace VR (Barco, Kortrijk, Holland), which allows the creation of a 'hologram' of the ultrasound image, depth perception and interaction with the rendered objects, an automated tool for fluid-filled spaces, sonography-based automated volume count (SonoAVC; GE Medical Systems), and extended imaging (XI) VOCAL™.[11,42] Ideally, 3D volumetry, like 2D parameters, should be measured using a standardized method to give reproducible results. However, different investigators have applied different techniques for 3D volumetry of different structures, and have presented different reference data (Table 1).
The aim of this article is to review the current measurement techniques for first-trimester volumetry and their potential applications.
Expert Rev of Obstet Gynecol. 2012;7(4):379-386. © 2012 Expert Reviews Ltd.