Ultrasound shows that isthmic contractions are common in second-trimester pregnancies and can affect cervical measurements, a Spanish study published July 20 in the American Journal of Obstetrics and Gynecology found.
Researchers led by Alba Farras, MD, PhD, from Vall d'Hebron University Hospital in Barcelona reported ultrasound findings that showed isthmic contractions occur in close to half of pregnant women after bladder voiding, and that it takes about 20 minutes for complete relaxation of the isthmus after a contraction.
"We recommend performing cervical assessment at least 20 minutes after bladder voiding to reduce the risk of bias in cervical length measurement and to avoid false images of placenta previa," Farras and colleagues wrote.
While asymptomatic contractions in the uterine isthmus -- also known as isthmic contractions --are a frequent physiological occurrence in pregnant women, the researchers noted that there is limited data on their prevalence and characteristics. Previous studies have used static ultrasound images, which may not sufficiently capture the nature of these contractions, according to the authors, so they used real-time ultrasound to investigate with an end goal of developing a new way to assess the contractions' presence and intensity.
The study included data from 30-second trimester singleton pregnancies; the women were assessed for isthmic contractions after voiding their bladders. The team found that the contractions occurred in 43% of the women and that the median time to complete isthmus relaxation was 19.7 minutes.
Farras and colleagues also found that relaxation tended to start with a decrease in muscular tone in the isthmus, creating a funnel-like image. This image disappeared as the isthmus continued to relax and uterine wall thickness decreased.
More research is needed to understand the role of isthmic contractions in pregnancy and birth, the authors urged. They recommended the following steps for assessing the presence of these contractions:
- Observe the cervix for at least 3 minutes. If the maximum isthmic length observed during that period is more than 18 mm, clinicians can assume that the patient is experiencing an isthmic contraction.
- If the patient is experiencing an isthmic contraction, wait at least 20 minutes and reassess the cervix.
- For universal cervical length screening performed in the second trimester of pregnancy, have the patient void their bladder, then scan the fetus, and perform the cervical length screening 20 to 30 minutes after voiding.
- Do not wait more than 30 minutes, because bladder filling can falsely exaggerate cervical length.
- Check for signs of isthmic contraction before diagnosing placenta previa.
- If the isthmic length is over 18 mm or subjective signs of contraction are seen, repeat the ultrasound examination 20 minutes later.
The entire study can be found here.
![Examples of ultrasound findings and techniques. (A) Images in a 39-year-old male patient with a mass in the left thigh. The mass is heterogeneous on the B-mode US image (compared with the patient in D) and showed increased microvascularity (superb microvascular imaging [SMI]) and shear-wave elastography (SWE) values. Undifferentiated pleomorphic sarcoma was diagnosed at biopsy (with pleomorphic rhabdomyosarcoma in surgical specimen). (B) Images in an 18-year-old male patient with a mass in the left leg. The mass is hypoechoic on the B-mode image, with no other findings suggestive of malignancy. The lesion is in contact with the cortex of the tibia, which is slightly irregular. CT revealed a doubtful anteromedial tibial erosion. The microvascular study demonstrated high vascularization, suggestive of malignancy. Periosteal Ewing sarcoma was diagnosed with both histologic and immunohistochemical confirmation. (C) Images in a 69-year-old female patient with a lump growing on the outside of the left leg. Multiple SWE examinations were performed (please note the high values obtained in the measurements, whereas the color map highlights the stiffness relative to adjacent tissues). SMI showed areas of increased vascularization to target for sampling. Undifferentiated spindle cell sarcoma was diagnosed at biopsy, with residual leiomyosarcoma in the surgical specimen after neoadjuvant therapy. (D) Images in a 56-year-old female patient with a mass in the right thigh. The mass is heterogeneous at both B-mode ultrasound (similar to patient A) and MRI (coronal T2-weighted spectral attenuated inversion recovery [SPAIR]; T1-weighted pre-contrast and postcontrast imaging), which even shows uptake after the administration of paramagnetic contrast material, which is traditionally suggestive of malignancy. Low values at SMI and elastography are suggestive of benignity. Spindle cell lipoma was diagnosed at biopsy, with atypical spindle cell lipomatous tumor in the surgical specimen.](https://img.auntminnie.com/mindful/smg/workspaces/default/uploads/2026/08/images-radiol250278fig2.APCFLSvX6p.jpg?auto=format%2Ccompress&dpr=2&fit=crop&h=167&q=70&w=250)
