Early first trimester uteroplacental flow and the progressive disintegration of spiral artery plugs: new insights from contrast-enhanced ultrasound and tissue histopathology.

Study question: Does the use of a vascular contrast agent facilitate earlier detection of maternal 22 flow to the placental intervillous space in the first trimester of pregnancy? 23 Summary answer: Microvascular filling of the intervillous space was demonstrated by contrast-24 enhanced ultrasound from 6 weeks of gestation onwards. 25 What is known already: During placental establishment and remodeling of maternal spiral 26 arteries, endovascular trophoblast cells invade and accumulate in the lumen of these vessels to 27 form ‘trophoblast plugs’. Prior evidence from morphological and Doppler ultrasound studies has 28 been conflicting as to whether the spiral arteries are completely plugged, preventing maternal 29 blood flow to the intervillous space until late in the first trimester. 30 Study design, size, duration: Uteroplacental flow was examined across the first trimester in 31 human subjects given an intravenous infusion of lipid-shelled octofluoropropane microbubbles 32 with ultrasound measurement of destruction and replenishment kinetics. We also performed a 33 comprehensive histopathological correlation using two separately archived uteroplacental tissue 34 collections to evaluate the degree of spiral artery plugging and evaluate remodeling of the 35 upstream myometrial radial and arcurate arteries. 36 Participants/materials, setting, methods: Pregnant women (n=34) were recruited in the first 37 trimester (range: 6+3 – 13+6 weeks gestation) for contrast-enhanced ultrasound studies with 38 destruction-replenishment analysis of signal intensity for assessment of microvascular flux rate. 39 Histological samples from archived in situ (Boyd Collection, n=11) and fresh first, second, and 40 third trimester decidual and post-hysterectomy uterine specimens (n=16) were evaluated by 41 immunohistochemistry and ultrastructural analysis. 42 3 Main results and the role of chance: Contrast agent entry into the IVS was visualized as early 43 as 6+3 weeks of gestation with some variability in microvascular flux rate noted in the 6 to 7+6 44 week samples. Spiral artery plug canalization was observed from 7 weeks with progressive 45 disintegration thereafter. Of note, microvascular flux rate did not progressively increase until 13 46 weeks which suggests that resistance to maternal flow in the early placenta may be mediated 47 more proximally by myometrial radial arteries that begin remodeling at the end of the first 48 trimester. 49 Limitations, reasons for caution: Gestational age was determined by crown rump length 50 measurements obtained by transvaginal ultrasound on the day of contrast-enhanced imaging 51 studies, which may explain the variability in the earliest gestational age samples due to the 52 margin of error in this type of measurement. 53 Wider implications of the findings: Our comprehensive in situ histological analysis, in 54 combination with the use of an in vivo imaging modality that has the sensitivity to permit 55 visualization of microvascular filling has allowed us to reveal new evidence in support of 56 increasing blood flow to the intervillous space from 6 weeks of gestation. Histologic review 57 suggested the mechanism may be blood flow through capillary sized channels that form through 58 the loosely cohesive ‘plugs’ by 7 weeks gestation. However, spiral artery remodeling on its own 59 did not appear to explain why there is significantly more blood flow at 13 weeks gestation. 60 Histologic studies suggest it may be related to radial artery remodeling, which begins at the end 61 of the first trimester. 6