Prenatal sampling techniques are integral to modern obstetrics, providing crucial insights into fetal health, facilitating early diagnosis of genetic disorders, and guiding medical interventions.

Abstract: Prenatal sampling techniques are integral to modern obstetrics, providing crucial insights into fetal health, facilitating early diagnosis of genetic disorders, and guiding medical interventions. This chapter focuses on three invasive procedures widely employed in prenatal care: Vesicocentesis, Thoracocentesis, and Fetal Blood Sampling (FBS). Vesicocentesis involves the ultrasound-guided puncture of the fetal bladder to sample urine, primarily for cases of fetal lower urinary tract obstruction (LUTO). Thoracocentesis addresses conditions such as fetal hydrothorax through ultrasound-guided drainage and sampling of fetal chest fluid. Fetal Blood Sampling (FBS),  performed for severe fetal anemia and other indications, entails the ultrasound-guided puncture of fetal vessels. Each procedure is meticulously discussed in terms of its indications, timing, equipment, risks, and clinical implications. Overall, these techniques are indispensable for diagnosing, staging, and managing various fetal conditions, with careful consideration of technique, timing, and interpretation aimed at optimizing outcomes and minimizing risks.

Keywords: Vesicocentesis, Thoracocentesis, Fetal Blood Sampling (FBS)

 Authors: Gareth J Waring1, Shohra Qaderi2
1.    Fetal Medicine Unit, Royal Victoria Infirmary, The Newcastle Upon Tyne NHS Foundation Trust, Newcastle Upon Tyne, NE1 4LP, United Kingdom
2.    Maternal Fetal Care Center, Division of Fetal Medicine and Surgery, Boston Children's Hospital, Harvard Medical School, Boston, MA, USA

Reviewers: Karen Fung-Kee-Fung, Alireza A. Shamshirsaz

View the Thoracocentesis Patient Information leaflet

View the Fetal Blood Sampling Patient Information leaflet

View the Vesicocentesis Patient Information leaflet


Prenatal sampling techniques are integral to modern obstetrics, providing crucial insights into fetal health, facilitating early diagnosis of genetic disorders, and guiding medical interventions. This chapter will summaries the evidence and approach required to perform the following three invasive procedures: Vesicocentesis, Thoracocentesis and Fetal Blood Sampling (FBS).

ICD code

J90      Pleural Effusion not elsewhere classified
J94.0   Chylous effusion



Vesicocentesis is the ultrasound guided needle puncture of the fetal bladder in order to sample fetal urine. 

Indications: Vesicocentesis can be indicated as part of a staging assessment of fetal lower urinary tract obstruction (LUTO). There are 3 principle uses for vesicocentesis and they are explained below. Whilst a full description of LUTO is beyond the scope of this document, briefly, it is obstruction of the bladder outflow usually due to posterior urethral valves or urethral atresia (1). While the severity of fetal diseases may vary widely, it's crucial to acknowledge the significant mortality and morbidity rates associated with higher stage conditions. In such cases, antenatal intervention can become necessary, often taking the form of vesicoamniotic shunting or fetal therapeutic cystoscopy. For instance, in terms of morphological considerations, the visualization of a "floppy" appearing bladder has been significantly linked to poorer infant outcomes when compared to fetuses exhibiting a full, rounded bladder shape (2). The act of draining the bladder can provide prognostic information to the operator based on time taken to refill (3). Analysis of fetal urinary biochemistry can help determine which fetus may benefit from an intervention in an attempt to preserve renal function as well as reduce the risk of lethal pulmonary hypoplasia (stage 2) and those whereby the benefit is only a reduced risk of lethal pulmonary hypoplasia (stage 3), (4). Some operators would also use this procedure to take a sample for genetic testing in cases of LUTO thereby avoiding the need for an additional placental biopsy or amniocentesis (5). 

Timing: The timing of the procedure will be determined by the time point at which intervention is being considered. Many cases of LUTO present following an initial first trimester dating and screening assessment (11-14 weeks). There is published data ranges for fetal urinary biochemistry from 13 weeks onwards (6). 

Equipment: The exact equipment required will vary depending on institution. A sterile field is required so the set up should be similar to what one would use for an amniocentesis. Local anaesthesia may be required depending on needle gauge. The needle should be either 20G of 22G. Syringes will be required to aspirate the fetal urine.

Risks: Given this is a sterile needle puncture procedure into a relatively large target the risks are likely to be small. The procedure related risk of miscarriage will  likely be higher than for an amniocentesis but lower than for a fetal blood sample therefore an empiric  1% risk is quoted locally, acknowledging  that there are no large series with reported complication rates in order to base these figures on (7). 

Procedure: Consent should be taken, and the patients’ abdomen should be cleaned and draped so the operator has a sterile field to work in. This operator would recommend local anaesthetic if using a 20G needle or anything larger. There should be continuous ultrasound guidance to direct the needle with the bladder visualised in the largest dimension. The operator should aim to enter the uterus avoiding the placenta and placing the needle into then lower aspect of the fetal bladder as the bladder will decrease in size with aspiration (Figure 1 and video 1). Colour doppler can be used to identify the umbilical arteries and avoid injury during the procedure. Once needle placement is confirmed within the bladder the inner stylet can be removed and aspiration can begin (Figure 2). 

Interpretation of results: As stated above appraisal of fetal urine should only be offered as part of a staging assessment for LUTO and/or for fetal genetic testing (see table 2). When using this staging approach, it is acknowledged that finding unfavourable urine biochemistry from one vesicocentesis does not define renal failure. Taken in isolation these analytes perform poorly in the prediction of poor postnatal renal function (8). The staging approach recommends serial sampling up to 3 times before the urinary analytes are accepted as unfavourable (Table 3). Analysis of the urinary analytes can help differentiate a stage 2 from stage 3 cases of LUTO whereby established renal damage is already present. It should be noted though that there is no compelling evidence that renal function will be preserved with either VAS placement or fetal cystoscopy in stage 2 cases (8). Repeat sampling can be considered from 48 hours after the first procedure (9). This also allows for assessment of bladder refilling. Fetal bladder volume can be measured prior to the first vesicocentesis and then assessed at 48 hours. A fetus with <27% bladder volume using the formula (current bladder volume-initial bladder volume/initial bladder volume) x100  are predicted to have stage 4/fetal renal failure (9).

Table 3 illustrates a scoring system published by Nassr et al using retrospective cases to further define a fetus that may benefit from intervention. It was found a score of >3 conferred a 100% mortality by 6 months. Systems such as this may hold potential utility when selecting cases, though at present there is no consensus as to which should be used (11). 



Thoracocentesis is ultrasound-guided needle puncture of the fetal chest. This is done to drain and sample a fetal pleural effusion.

Indications: Thoracocentesis can be performed in cases of large pleural effusions/hydrothorax. It can be a diagnostic or therapeutic procedure. A fetal hydrothorax can be either primary (isolated) or secondary (with other fetal abnormalities). With primary hydrothorax the concern is compression of the fetal lungs causing pulmonary hypoplasia. A small hydrothorax without hydrops can be monitored and may simply resolve (10). However, in the setting of a rapidly enlarging hydrothorax with mediastinal shift, fetal hydrops or significant polyhydramnios, drainage is indicated. Decompression of the chest cavity can allow the lungs to re expand. This can reduce the chance of pulmonary hypoplasia as well as improve venous return to the heart through reversal of mediastinal shift and potential improvement in hydrops. From a diagnostic perspective, if the lungs do not expand following drainage this may increase the risk of pulmonary hypoplasia. If hydrops does not resolve, then other underlying causes should be considered. Pleural fluid can be sent off for cytogenetic analysis and cell count. The drainage of fluid from the chest can also improve visualisation of the structures of the thorax and the heart (11). There is a high chance of re-accumulation of fluid following thoracocentesis. The risk of this is a high as 75% (10). For this reason, as a therapeutic procedure many operators would place a pleuroamniotic shunt (PAS) if fluid does reaccumulate or opt for PAS from the outset, particularly in cases with hydrops (12-14). The fluid will generally re accumulate within the following 48 hours. 

Timing: The timing of the procedure will be determined by the time point at which intervention is being considered. A hydrothorax can appear at any point during a pregnancy. Whilst cystic hygromas in the first trimester can be complicated with a secondary hydrothorax in the setting of hydrops, a primary hydrothorax most commonly presents in the second or early third trimester (15). Thoracocentesis is a particularly attractive option below 21 weeks when PAS is technically more difficult and higher risk(13).

Equipment: The set up for this procedure should be similar to an amniocentesis. A sterile field is required. The principle remains that the smallest gauge needle to achieve the goal should be used, this would typically be a 22 or 20 gauge. Needle size will influence the operator decision as to whether to use local anaesthetic. The operator should keep in mind the distance between the maternal skin and the fetal chest will usually be longer than would be the case for an amniocentesis, therefore a longer needle may be required. 

Risks: Given this is a sterile needle puncture procedure into the fetal chest via the amniotic cavity the risks are likely to be small. The procedure-related risk of miscarriage will be likely higher than for an amniocentesis but presumably lower than for a fetal blood sample therefore a 1% procedural risk is quoted locally with a similar rate for rupture of membranes. The additional considerations  that should be noted on the consent for this procedure is the chance of the pleural fluid recurring  and the need for further intervention (12). Procedure: Consent should be taken, and the patient’s abdomen should be cleaned and draped so the operator works within a sterile field. This operator would recommend local anaesthetic if using a 20G needle or anything larger. There should be continuous ultrasound guidance to direct the needle with the fetal chest, visualised ideally in the coronal plane. The operator should aim to enter the uterus avoiding the placenta and placing the needle into the fetal chest in the mid axillary line avoiding the fetal ribs (Figure 3 and video 2). Care should be taken to avoid the fetal heart upon entering the chest. Once needle placement is confirmed within the chest the inner stylet can be removed and aspiration can begin. Pleural fluid can be sent for a cell count and cytogenetic testing. Cytogenetic testing can also be performed on amniotic fluid taken at the time (video 3).  

Interpretation of results: With aspiration the operator may immediately notice re expansion of the fetal lung and reduction in the mediastinal shift. If there is no lung expansion then this may reflect pulmonary hypoplasia and predict a poor outcome (16). With the fluid removed the operator has the opportunity to perform a fetal ECHO with potentially improved views. Full chromosomal analysis should be offered as there is ~5% risk of aneuploidy (17). Finally, cell count can be assessed, in an attempt to ascertain the underlying aetiology but this is a disputed field. Some authors describe a lymphocyte count of >80% as pathognomonic for chylothorax (18).   

Fetal blood sampling

This describes the ultrasound-guided needle puncture to evaluate a sample of fetal blood, typically accessed via the umbilical vein, the cord insertion at the placenta, a free loop of cord or the fetal intrahepatic vein (IHV). 

Indications: The most common indication for a Fetal Blood Sampling (FBS) is the diagnosis of severe and moderate fetal anaemia. Fetal anaemia should be suspected in high-risk groups. The most common cause of severe fetal anaemia is maternal anti-D alloimmunisation. There are other maternal antibodies that can cause this such as anti-K and anti-c. Often these women will be identified as carrying the antibodies from early pregnancy and level of risk can be ascertained through; antibody titres, paternal blood group testing and free fetal DNA analysis for fetal blood group (19). Certain fetal infections are associated with anaemia like parvovirus B-19 and cytomegalovirus. Severe fetal anaemia can be screened for by measuring the peak systolic velocity in the middle cerebral artery. A PSV >1.5 MoMs will screen positive for moderate and severe fetal anaemia with 100% sensitivity and a 12% false positive rate (20). Anaemia may also be suspected when a fetus has hydrops. Whilst the level of anaemia required to cause hydrops is unpredictable it has been shown to frequently occur when fetal haemoglobin <7 g/dl/Haematocrit <20% (21). When a FBS is performed for fetal anaemia, an operator will usually be prepared to transfuse the fetus immediately once anaemia is confirmed to avoid the risks of multiple entries. The other indications for FBS are either less well established or now no longer indicated. FBS was a key tool in the past diagnosis and treatment of fetal neonatal alloimmune thrombocytopenia (FNAIT). Because the diagnosis of an ‘at risk’ fetus can be confirmed via amniocentesis and the subsequent treatment has been revolutionised by the use of intravenous immunoglobulin there is a very limited role for FBS and subsequent platelet transfusions anymore (22). It should be noted that some authors would support FBS at 32 weeks in patients wishing to pursue a vaginal birth (if platelet count is >100x109/L then trial of labour would be permissible) (23). Fetal platelet count has also been assessed as part of a workup for a fetus with congenital CMV. It is unclear that knowledge of fetal platelet count in this setting will significantly alter management and therefore there are no strong recommendations for FBS to be used in this group (24). Whilst fetal blood can be taken for chromosomal analysis, the increased risks of FBS vs amniocentesis limits the role of FBS in this circumstance to when access to the fetal circulation is undertaken for another reason such as investigation and treatment of fetal anaemia or feticide.

FBS has been described in the assessment of fetal renal function in cases of lower urinary tract obstruction. Assessment of serum β-2 microglobulin performs similarly to urinary β-2 microglobulin in the prediction of postnatal renal outcome (7). However, its role is most likely limited to cases later in pregnancy where vesicocentesis appears to be a more challenging procedure such as anterior placenta with a small bladder after 20 weeks. Please see the section on vesicocentesis for more information on the assessment of fetal renal function in cases of LUTO.  

Timing: Exact timing will be dependent on when the intervention is being considered. FBS can be undertaken from 16 weeks though there is no doubt this procedure is technically more challenging below 22 weeks (see discussion of risks below).

Equipment: Operators will require skin preparation to ensure a sterile field. This operator would recommend chlorhexidine (2%) with ethanol (70%). Local anaesthesia is likely to be required here to reduce maternal discomfort but ultimately this is the decision of the operator and may depend on gauge of needle used. Needle gauge and length will be dependent on the subject. Early FBS with smaller vessels may require a 22G needle. Alternatively, if performed later or with large maternal pannus then a 20G needle may be preferable to improve visualisation, reduce bending and promote a quicker procedure if giving a large volume tranfusion. If fetal paralysis is planned there is a good rationale to give atracurium at a dose of 0.4mg/kg injected into the umbilical vein or IHV (see procedure notes). Given this procedure will most likely be undertaken for suspected anaemia the operator will also require access to a point of care test to establish the degree of anaemia and plan treatment. The teams carrying out FBS/IUT will require syringes for aspiration of fetal blood.  

Risks: Risks should be considered as early and late. 

The most concerning early procedure related complication is fetal loss, this is defined as loss within 2 weeks of the procedure. In general this is rare with a rate of 1-2% being reflected in the larger series (25). However, there are features that make fetal loss more common. Early gestation at FBS (and usually subsequent IUT) carries a fetal loss rate of 10% (17). Hydrops also is associated with a higher risk of fetal loss following FBS. This ranges between 10-15% but also increases with decreasing gestation and some small series quote 25% (26).  Another early complication is the need for emergency CS. Clearly this is only applicable at later gestations and indicated in the setting of fetal distress. The fetal heart rate should be monitored during an FBS (and subsequent IUT) and post-procedure. Fetal distress can be caused by local cord accidents (rupture, excessive bleeding, compression from a haematoma, vessel spasm) or volume overload. Most episodes of fetal bradycardia during FBS and subsequent IUT will settle with cessation of IUT and recommencement at a slower rate or repositioning of the needle if inadvertent umbilical artery puncture (27). Overall the risk of emergency CS is 1-2% (17). Rupture of membranes is uncommon (<1%) as is infection (<1%), (17).

A late procedural-associated risk appears to be related to IUT in the setting of maternal alloimmunization. Following transplacental access up to 25% of patients will demonstrate the presence of new, additional red cell antibodies (30).  Overall though  the  perinatal survival following IUT for RBC alloimmunized pregnancies is in the order of  82.4% (28). 

Procedure: The location to perform an FBS is at the discretion of the operator and institutional protocols. There is no specific reason to consider the operating theatre a better place to do this than the scan room. Clearly if there is felt to be a high chance of needing emergent delivery this should be factored into planning (29). The use of corticosteroids for antenatal optimisation is also left to operator preference, given the risk of needing emergency delivery is low. Given that steroids are most useful for optimising fetal lungs if given as a single course with delivery undertaken within 7 days of the 2nd dose, many operators would not routinely give them. The patient’s abdomen should be cleaned and draped to ensure a sterile field. Most operators would advocate local anaesthetic. The operator would then use a 20G or 22G needle to take the sample (see equipment section to see why a certain size gauge may be chosen), (figure 4 and video 4). The operator then needs to choose the most appropriate site to take the FBS. This will be dependent on factors such as placental position, cord insertion, gestation and aim of FBS. With an anterior placenta and favourable cord insertion many operators would aim to take the FBS from the placental cord insertion via cannulation of the umbilical vein. The advantage of cannulating here is the cord is relatively stable and therefore the needle is less prone to become dislodged. It is also associated with a shorter procedure time when compared with puncture of a free loop of cord (30). There is a small risk of maternal contamination using this route (2%). A free loop of cord can be used for FBS but is not typically advised if the plan is for an IUT given the increased risk of needle dislodgement. With a posterior placenta most operators would choose fetal intrahepatic vein (IHV), (25). This carries a minimal risk of sample contamination. It also has been favoured for earlier transfusions (17). With IHV puncture the needle can be prone to dislodgement from the vessel with vigorous fetal movement therefore in planning a IUT, particularly if transfusing a large volume, operators may wish to consider induction of fetal paralysis. Atracurium can be injected into the fetal circulation via the chosen access point once the operator confirms they are in the vessel. Whilst there are other agents that can be used, atracurium is not eliminated by the fetal liver and hence may be particularly useful with hydrops. Additionally, the breakdown products do not have significant cardiovascular or neuromuscular effects. When compared with pancuronium fetal movements returned quicker (median 24 vs 57 minutes) and there was no reduction in fetal heart rate variability directly after IUT (31). A large series suggested a reduced risk of procedure related complications in cases that use fetal paralysis vs those in which it was not used (25).

Cardiocentesis is typically reserved for FBS taken at the time of a feticide procedure or as a place of last resort for an IUT. Once the needle has been successfully placed into left ventricle blood is aspirated into a syringe and required samples can be taken (32, 33, 34). The needle can be left in placed whilst a point of care test is performed and flushed with saline and if appropriate an IUT can then commence. Once the procedure is complete the needle should be withdrawn, and the puncture site observed for streaming (video 5). When FBS is performed after viability most operators would rescan after a short period of time (e.g. 30 minutes) to ensure no immediate problems. 

Interpretation of results: If the FBS is being performed to assess for fetal anaemia then bedside point of care testing for degree of fetal anemia is required. Operators will typically assess either fetal haemoglobin (Hb) or fetal haematocrit (Hct). Operators will take note of this value alongside the expected fetoplacental volume for the given gestation when deciding how much blood to give at an IUT. There are various papers as well as freely available online calculators to help with this (34).

If FBS is being undertaken to assess fetal platelet count, either due to FNAIT or alongside fetal Hb/Hct in cases of anaemia secondary to suspected viral infection, then interpretation will be down to the operator. As noted above some operators may perform FBS at 32 weeks in patients that may wish a trial of labour in cases of FNAIT. Here they would look for a fetal platelet count >100x109/L (23). In cases of fetal thrombocytopenia secondary to viral infections such as parvovirus or CMV then an operator may consider a platelet transfusion at ‘severe’ levels typically recognised as < 50x109/L. Its recommend that platelets should be readily available for transfusion during any Fetal Blood Sampling (FBS) procedure, particularly in cases of suspected or confirmed fetal hParvo-B19 infection. This is especially crucial for cases diagnosed at or beyond 22 weeks’ gestation or when severe fetal anemia is suspected based on Middle Cerebral Artery Peak Systolic Velocity (MCA-PSV) Doppler ultrasound scan results. Additionally, it’s important to consider the potential decrease in fetal platelet concentration following Red Blood Cell (RBC) transfusion in these scenarios (35). 

If operators are using FBS to assess fetal renal function, then a serum β-2 microglobulin of >5mg/L has been used to try and distinguish poor from favourable renal outcomes in cases of LUTO. It should be noted that when this test is performed its sensitivity is better the nearer to delivery it is performed. When sampled sequentially it was shown that sensitivity in the first sample was 64.3% vs 96.4% for the last sample (35). 


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The article should be cited as: Waring G, Qaderi S: Fetal Sampling Techniques: Blood, Urine, and Tissue Biopsy, Visual Encyclopedia of Ultrasound in Obstetric and Gynecology,, June 2024.

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