General principles of Ultrasound-Guided Procedures

In utero ultrasound-guided procedures involve medical interventions performed on a developing fetus inside the mother’s womb with the assistance of real-time ultrasound imaging. These techniques enable diagnosis, management and sometimes treatment of various fetal conditions, including genetic and infectious diseases and fetal malformations. The generalization and improvements of in-utero invasive testing have been closely related to the evolution and refinement of ultrasound technology. With the subsequent development of invasive therapeutic techniques, they have revolutionized antenatal care for affected fetuses. In the 1980-1990s, invasive testing, including amniocentesis, chorionic villus sampling and cordocentesis (fetal blood sampling from the umbilical cord) were increasingly performed and genetic and biological data gleaned from access to fetal tissues  improved the assessment of fetal health and  in many cases, altered perinatal prognosis (1,2). Advancements in imaging technology, surgical techniques, and anesthesia led to a shift from a diagnostic perspective only towards more global fetal care, with therapeutic interventions. Procedures such as ultrasound-guided fetal transfusion, fetal shunt placement and fetoscopic surgery rose progressively to become what nowadays are  standard procedures (3–5). Ultrasound-guided invasive tests have since become safe routine prenatal techniques (6–8) and fetal interventions are constantly improving in efficiency and safety (9,10).

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While each technique has its own specific characteristics, it is essential to emphasize common aspects shared by all procedures.

Before the procedure: importance of prenatal counseling 
Before any procedure, a full morphological assessment of the fetus must be performed by a dedicated specialist. The prenatal parental counseling is essential and allows an informed decision-making process regarding pregnancy management. It should include: 
-    Description and details about ultrasound findings and their consequences.
-   Clarification of the procedure’s purpose: either the detection of genetic, chromosomal abnormalities, or infections in case of diagnostic procedures; or potential improvement of the prognosis for therapeutic procedures.
-    Description of the procedure: the timing, the technical aspects, duration, and post-intervention surveillance should be described.
-    The benefits and risks of the procedure, particularly regarding miscarriage or preterm delivery, or procedure failure. 

Common technical aspects
Rhesus prophylaxis: invasive procedures may induce feto-maternal hemorrhage, a cause of maternal rhesus immunization in RhD-negative women carrying a RhD-positive fetus. Prophylaxis with anti-D antibodies is known to decrease this risk. While a recent study reported very low risk of rhesus immunization in diagnostic procedures (11), Rhesus prophylaxis is currently recommended after invasive procedures (diagnostic or therapeutic) in non-sensitized Rhesus-negative women with a Rhesus negative fetus assessed by cffDNA testing, or in cases of unknown fetal Rhesus status (12).

Prevention of infections: Before invasive diagnostic procedures, such as transabdominal CVS, amniocentesis or FBS, the abdominal skin is cleaned with an antiseptic solution, iodine, or chlorhexidine according to local protocols. The use of sterile drape with aperture to protect the sterile field  is commonly employed and the probe is covered by a sterile bag. Aseptic principles are reinforced in cases of therapeutic interventions, particularly with fetoscopic surgeries. While antibiotic prophylaxis is not recommended before invasive diagnostic procedures (13), per-operative administration of antibiotic (mainly Cefazoline) is largely admitted for fetal invasive therapy (14–17).
The frequency of perinatal complications following an invasive procedure (bleeding, preterm rupture of membranes, infection, or preterm delivery) depends on the clinical context, indication, and the type of intervention. The introduction of new additional therapies may be considered after evaluation of such risks compared to the potential benefit of these interventions

Diagnostic procedures are now common prenatal interventions, performed in outpatient facilities, with low risk of complication when performed by trained specialists (7). There are three main diagnostic procedures:

Amniocentesis: Amniocentesis involves using ultrasound guidance to insert a thin needle through the mother's abdomen into the amniotic sac to obtain a sample of amniotic fluid. Under continuous ultrasound guidance, a 20 – 22-G needle is inserted transabdominally in the amniotic cavity with visualization of the entire needle’s trajectory (12,18,19). Whenever possible, a transplacental entry (through the placenta) should be avoided. Once the needle has reached the amniotic sac, the stylet can be removed to obtain a sample of amniotic fluid (figure 1). Amniocentesis should be performed after 15 weeks of gestation, due to an increased risk of complications earlier than this stage.

Chorionic Villus Sampling (CVS): CVS is a procedure used to extract a small sample of trophoblastic cells (chorionic villi) under ultrasound guidance. This tissue sample is then tested for genetic or chromosomal abnormalities, or infection (19). The procedure is generally performed between 11 and 14+6 weeks of gestation. Either a transabdominal or transvaginal approach is employed. Choice of approach may be determined by placental position and ease of access to the placenta  or by operator preference. Using the transabdominal approach, either a single needle of 17 – 20 G or a two-needle set of outer 17/19 G and inner 19/20 G are inserted into the placenta (20). Once the needle has reached its target, back-and-forth movements are performed to extract trophoblastic cells with a syringe under vacuum pressure (figure 2). The transabdominal access to the placenta may be challenging in case of posterior location with retroverted uterus. Alternatively, a transcervical approach may be considered (21). For further discussion see VISUOG chapter on Invasive Diagnostic Testing in Obstetrics: Amniocentesis and Chorionic Villus Sampling.

Fetal Blood Sampling (FBS): Fetal blood sampling refers to an ultrasound-guided puncture of the umbilical vein to obtain a small sample of fetal blood. A 20–G needle is introduced transabdominally under continuous ultrasound guidance and inserted into the umbilical vein. The site of the puncture depends on the placental location and accessibility. Placental cord insertion is generally preferred, especially in case of anterior placenta. When the placenta is posterior, or the cord insertion is not reachable, it is advised to puncture a free loop. Once the needle has reached its target, a small sample of fetal blood is aspirated. The procedure can be performed after 18 weeks of gestation (12,22,23). 

The use of local anesthetics is currently not recommended for amniocentesis and fetal blood sampling, as both procedures generate minor pain. The use of local anesthetics may be offered in CVS, particularly in case of two-needle technique since the needle used is larger than the one used for amniocentesis and FBS (12,24,25).
After the procedure, a simple limitation of physical activity for 12–24 hour is commonly recommended, although there is no evidence of clinical benefit. There is no current recommendation about the additional use of pharmacological treatment, as no benefit has been identified (12). For further discussion see VISUOG chapter on Fetal Sampling Techniques:  Blood, Urine and Tissue Biopsy. 
 

The use of real-time ultrasound imaging plays a crucial role in therapeutic procedures, to guide the placement of instruments and to monitor the fetus throughout the whole intervention. Although most of those procedures are performed with a minimally invasive approach, they usually require additional therapies, including maternal and fetal anesthesia and tocolytic administration. 
The management of maternal anesthesia depends on the type of procedure, patient characteristics and local guidelines. Most minimally invasive therapeutic procedures are performed with either intravenous sedation supplemented with local anesthetic or regional anesthesia (26). Similarly, most therapeutic procedures require an administration of sedation to the fetus, to minimize the pain and discomfort caused by the intervention and to reduce the complications and difficulties of the procedure caused by fetal movements. Fetal anesthetic is either administered through the umbilical vein or directly by an intramuscular injection. Drugs may include anticholinergic, opioid and nondepolarizing paralytic, depending on the procedure and local protocols (26–28).     
In addition to assuring acute well-being of the fetus following an invasive ultrasound-guided procedure, the main obstetrical concerns are preterm delivery and preterm premature rupture of membranes. To reduce the risk of uterine contractility and therefore, the risk adverse outcomes, tocolytics are frequently administered (29,30).
Two of the most common procedures includes fetal shunt placements and in- utero fetal transfusion.

Fetal shunt placement: the main indication for shunt placement is pleural effusion. Briefly, under continuous ultrasound guidance and after fetal anesthesia, a trocar is inserted into the pleural effusion, preferably with a posterior entry (figure 3a and 3b). A double pigtail catheter is then inserted through the cannula, depositing the distal end in the effusion and the proximal end in the amniotic fluid (15,31). Thoraco-amniotic shunt placement is mainly indicated in case of signs of poor hemodynamic tolerance, compression, mediastinal deviation. Other indications of fetal shunt placement include vesicoamniotic shunting for lower urinary tract obstruction, and cystic pulmonary airway malformations. For further discussion see VISUOG chapter on Ultrasound-Guided Fetal Surgeries:Techniques and Applications

In-utero transfusion: In-utero transfusions are performed in case of severe anemia, mainly caused by feto-maternal red cell incompatibility and parvovirus B19 fetal infections. The initial approach is similar to a fetal blood sampling. After introducing a 20-G needle under continuous ultrasound guidance into the umbilical vein, usually at the cord insertion site  in an anterior placenta or into the intra-hepatic portion of the umbilical vein in cases of posterior placentation. fetal blood is administered progressively. The volume depends on both the severity and gestational age (27,32).For further discussion see VISUOG chapter on Intrauterine Fetal Transfusion: Indications and Procedure.

In utero ultrasound-guided procedures mark a significant advancement in prenatal medicine, enabling precise diagnosis and management of fetal conditions. Evolution in ultrasound technology revolutionized these interventions, transitioning from diagnostics to comprehensive fetal care. Diagnostic procedures like amniocentesis and fetal blood sampling have become routine, performed with high precision and safety. Therapeutic interventions, including shunt placements, in-utero transfusions, or fetoscopic surgeries also increase in precision, safety, and efficiency. Standardized protocols for pre-procedural counseling of the parents-to-be, informed consent, technical execution, and post-intervention care underscore the importance of a structured approach. With constant advancements, these procedures continue to enhance fetal health.

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