Degree of fetal umbilical venous constriction at the abdominal wall in a low-risk population at 20-40 weeks of gestation

The Development of the Umbilical Vein and Its Anatomical and Clinical Significance

The umbilical vein is one of the most essential vessels in the human embryo. Anatomical structures though may vary in several cases. During the fourth and eighth weeks of gestation, the umbilical cord is formed. Initially, two umbilical arteries and veins exist. During development, the obliteration of the right umbilical vein occurs. The fetus and its liver receive macronutrients and oxygen from the placenta via the umbilical vein, which primarily supplies the left lobe of the liver before branching into the left portal vein and the ductus venosus. The ductus venosus directs blood from the umbilical vein directly into the systemic circulation through the inferior vena cava and right atrium, bypassing the fetal liver. In some cases, variations are observed. Disorders of the umbilical veins may involve the persistence of embryological structures, abnormal insertion or course, and the presence of supernumerary vessels. For example, the persistence of the right umbilical vein, duplication of the umbilical vein, and umbilical vein varix are some important variations to acknowledge in order to be able to understand the potential outcomes of the newborn. The majority of venous system anomalies are rare, and some may remain completely asymptomatic. Different forms of umbilical cord abnormalities, however, may be potentially fatal or pose a serious threat to fetal health. Therefore, clinically, early detection of these malformations is highly important in order to make a proper diagnosis and management of care. The aim of this study is to acknowledge the different types of umbilical vein variations through its development and its relation with liver parenchyma in order to achieve a better understanding and planning in surgical interventions. An advanced review search of the literature was undertaken. The literature review was conducted using the search engine of the PubMed database and Google Scholar. The years included in data collection were 1960-2022. All articles that met the inclusion criteria were taken under consideration.

Structural and functional fetal cardiac imaging using low field (0.55 T) MRI

Purpose

This study aims to investigate the feasibility of using a commercially available clinical 0.55 T MRI scanner for comprehensive structural and functional fetal cardiac imaging.

Methods

Balanced steady-state free precession (bSSFP) and phase contrast (PC) sequences were optimized by in utero studies consisting of 14 subjects for bSSFP optimization and 9 subjects for PC optimization. The signal-to-noise ratio (SNR) of the optimized sequences were investigated. Flow measurements were performed in three vessels, umbilical vein (UV), descending aorta (DAo), and superior vena cava (SVC) using the PC sequences and retrospective gating. The optimized bSSFP, PC and half-Fourier single shot turbo spin-echo (HASTE) sequences were acquired in a cohort of 21 late gestation-age fetuses (>36 weeks) to demonstrate the feasibility of a fetal cardiac exam at 0.55 T. The HASTE stacks were reconstructed to create an isotropic reconstruction of the fetal thorax, followed by automatic great vessel segmentations. The intra-abdominal UV blood flow measurements acquired with MRI were compared to ultrasound UV free-loop flow measurements.

Results

Using the parameters from 1.5 T as a starting point, the bSSFP sequences were optimized at 0.55 T, resulting in a 1.6-fold SNR increase and improved image contrast compared to starting parameters, as well as good visibility of most cardiac structures as rated by two experienced fetal cardiologists. The PC sequence resulted in increased SNR and reduced scan time, subsequent retrospective gating enabled successful blood flow measurements. The reconstructions and automatic great vessel segmentations showed good quality, with 18/21 segmentations requiring no or minor refinements. Blood flow measurements were within the expected range. A comparison of the UV measurements performed with ultrasound and MRI showed agreement between the two sets of measurements, with better correlation observed at lower flows.

Conclusion

We demonstrated the feasibility of low-field (0.55 T) MRI for fetal cardiac imaging. The reduced SNR at low field strength can be effectively compensated for by strategically optimizing sequence parameters. Major fetal cardiac structures and vessels were consistently visualized, and flow measurements were successfully obtained. The late gestation study demonstrated the robustness and reproducibility at low field strength. MRI performed at 0.55 T is a viable option for fetal cardiac examination.

A new classification of congenital abnormalities of UPVS: sonographic appearances, screening strategy and clinical significance

The umbilical–portal venous system (UPVS) plays an important role in embryonic development, as well as a significant blood circulation system to ensure the normal blood supply of fetal heart and brain and other vital organs. Congenital anomalies of UPVS contain many subtypes with a broad spectrum of manifestations and prognoses. Furthermore, because of fetal small lumen of UPVS, the sonographic evaluation remains difficult in utero. Appreciation of normal embryology and anatomy of UPVS is essential to an understanding of sonographic characteristics of anomalies of UPVS and fetal sequential changes. Through reviewing previous references and our experience with congenital abnormalities of UPVS, a new comprehensive classification is proposed. The new classification identifies three types of congenital abnormalities of UPVS based on morphological abnormalities and shunts. The embryology and etiology, sonographic, clinical and prognostic characteristics of each subtype of the new classification are described in detail. Knowledge of congenital abnormalities of UPVS can give sonographers a clue and aid prenatal sonographic diagnosis. The purpose of this article is to help the sonographers to understand the new classification of congenital abnormalities of UPVS, master the sonographic characteristics of each subtype and prenatal ultrasonographic screening strategy, and guide subsequent appropriate counseling and management.

Placental Corticotrophin-Releasing Hormone is a Modulator of Fetal Liver Blood Perfusion

CONTEXT

Variation in fetal liver blood flow influences fetal growth and postnatal body composition. Placental corticotrophin-releasing hormone has been implicated as a key mediator of placental-fetal perfusion.

OBJECTIVE

To determine whether circulating levels of placental corticotrophin-releasing hormone across gestation are associated with variations in fetal liver blood flow.

DESIGN

Prospective cohort study.

METHODS

Fetal ultrasonography was performed at 30 weeks' gestation to characterize fetal liver blood flow (quantified by subtracting ductus venosus flow from umbilical vein flow). Placental corticotrophin-releasing hormone was measured in maternal circulation at approximately 12, 20, and 30 weeks' gestation. Multiple regression analysis was used to determine the proportion of variation in fetal liver blood flow explained by placental corticotrophin-releasing hormone. Covariates included maternal age, parity, pre-pregnancy body mass index, gestational weight gain, and fetal sex.

RESULTS

A total of 79 uncomplicated singleton pregnancies were analyzed. Fetal liver blood flow was 68.4 ± 36.0 mL/min (mean ± SD). Placental corticotrophin-releasing hormone concentrations at 12, 20, and 30 weeks were 12.5 ± 8.1, 35.7 ± 24.5, and 247.9 ± 167.8 pg/mL, respectively. Placental corticotrophin-releasing hormone at 30 weeks, but not at 12 and 20 weeks, was significantly and positively associated with fetal liver blood flow at 30 weeks (r = 0.319; P = 0.004) and explained 10.4% of the variance in fetal liver blood flow.

CONCLUSIONS

Placental corticotrophin-releasing hormone in late gestation is a possible modulator of fetal liver blood flow and may constitute a biochemical marker in clinical investigations of fetal growth and body composition.

Variation of the maximum velocity along the umbilical vein supports the Reynolds pulsometer model

OBJECTIVE

To challenge, with a modern sonographic approach and a numerical model, the Reynolds's concept which suggests that the vascular structure of the umbilical cord could act as a pulsometer facilitating the venous return to the foetus.

METHOD

Forty-five patients between 20 and 28 weeks of gestation were included in the study. The blood maximum velocity in the umbilical vein, measured at both foetal and placental ends, was assessed. Several sonographic parameters of the cord, including the diameter of the umbilical vein at both extremities, cord cross-sectional area and Wharton's jelly section surface were measured. We compare our data with those of a numerical model.

RESULTS

A difference in maximum velocity between the two extremities of the umbilical vein (Δ^UVV_max) was noted. The maximum velocity was significantly higher at the foetal umbilical end (14.12 +/-3.18 cm/s) than at the placental end (11.93 +/-2.55 cm/s; p < 0.0001). The mean difference is 2.2 +/- 2.3 cm/s. No difference in the umbilical vein diameter was measured at both cord ends (umbilical 4.85 +/-0.9 mm, placental 4.86 +/-0.87 mm, p < 0.0001). There is no significant relationship between Δ^UVV_max and the cord cross-sectional area or Wharton's jelly index.

CONCLUSION

Modifications of the spatial velocity profile together with the pulsometer model could explain the maximum velocity changes that is measured in the umbilical vein along the cord. This numerical model consolidates the sonographic observations.

Artificial Placenta - Lung Assist Devices for Term and Preterm Newborns with Respiratory Failure

Respiratory insufficiency is a major cause of neonatal mortality and long-term morbidity, especially in very low birth weight infants. Today, non-invasive and mechanical ventilation are commonly accepted procedures to provide respiratory support to newborns, but they can reach their limit of efficacy. To overcome this technological plateau and further reduce mortality rates, the technology of an “artificial placenta”, which is a pumpless lung assist device connected to the umbilical vessels, would serve to expand the therapeutic spectrum when mechanical ventilation becomes inadequate to treat neonates with severe respiratory insufficiency.

The first attempts to create such an artificial placenta took place more than 60 years ago. However, there has been a recent renaissance of this concept, including developments of its major components like the oxygenator, vascular access via umbilical vessels, flow control, as well as methods to achieve hemocompatibility in extracorporeal circuits. This paper gives a review of past and current development, animal experiments and human case studies of artificial placenta technology.

Stenosis of alternative umbilical venous pathways in absence of the ductus venosus

OBJECTIVE

We evaluated fetuses with absence of the ductus venosus (ADV) and restricted alternative umbilical venous pathways.

METHODS

We identified 3 cases that fit our objective. The angles of insonation for spectral Doppler ultrasound interrogation were less than 20 degrees in all cases. We used commercially available ultrasound systems with a curved array transducer.

RESULTS

In all 3 cases, we noted mild cardiac volume overload without fetal hydrops.

CONCLUSIONS

We speculate that the fetus with ADV and a restrictive alternative umbilical venous pathway may have a more benign clinical course than fetuses previously reported with unrestricted alternative pathways.

Detection of umbilical venous constriction by Doppler flow measurement at midgestation

OBJECTIVES

To investigate whether umbilical venous velocity and venous velocity pulsation are associated with umbilical vein diameter, umbilical ring diameter and umbilical cord coiling index at midgestation.

METHODS

Two hundred and eighty pregnant women were enrolled in the study at between 18 and 24 weeks of gestation. The diameter of the umbilical cord and internal diameter of the umbilical vein in a free loop and at the ring, and the umbilical coiling index, were measured using ultrasonography. Umbilical venous velocities were measured by Doppler ultrasonography at the umbilical ring and a free loop of the cord.

RESULTS

All variables were successfully measured in 92% of the patients. There were negative correlations between the diameters of the umbilical ring and of the umbilical vein at the ring and the venous velocity at the umbilical ring. The venous velocity at the umbilical ring was significantly higher and the umbilical ring diameter was significantly lower in fetuses with umbilical venous pulsation at the free loop. Significant correlations were observed between the venous velocity and amplitude of pulsation. Venous pulsations at the free loop were frequently observed in fetuses with a hypercoiled cord.

CONCLUSION

High venous velocity and increased venous pulsation at the umbilical ring may be associated with umbilical cord constriction.

Where the O2 goes to: preservation of human fetal oxygen delivery and consumption at high altitude

Fetal growth is decreased at high altitude (> 2700 m). We hypothesized that variation in fetal O(2) delivery might account for both the altitude effect and the relative preservation of fetal growth in multigenerational natives to high altitude. Participants were 168 women of European or Andean ancestry living at 3600 m or 400 m. Ancestry was genetically confirmed. Umbilical vein blood flow was measured using ultrasound and Doppler. Cord blood samples permitted calculation of fetal O(2) delivery and consumption. Andean fetuses had greater blood flow and oxygen delivery than Europeans and weighed more at birth, regardless of altitude (+208 g, P < 0.0001). Fetal blood flow was decreased at 3600 m (P < 0.0001); the decrement was similar in both ancestry groups. Altitude-associated decrease in birth weight was greater in Europeans (-417 g) than Andeans (-228 g, P < 0.005). Birth weight at 3600 m was > 200 g lower for Europeans at any given level of blood flow or O(2) delivery. Fetal haemoglobin concentration was increased, decreased, and the fetal / curve was left-shifted at 3600 m. Fetuses receiving less O(2) extracted more (r(2) = 0.35, P < 0.0001). These adaptations resulted in similar fetal O(2) delivery and consumption across all four groups. Increased umbilical venous O(2) delivery correlated with increased fetal O(2) consumption per kg weight (r(2) = 0.50, P < 0.0001). Blood flow (r(2) = 0.16, P < 0.001) and O(2) delivery (r(2) = 0.17, P < 0.001) correlated with birth weight at 3600 m, but not at 400 m (r(2) = 0.04, and 0.03, respectively). We concluded that the most pronounced difference at high altitude is reduced fetal blood flow, but fetal haematological adaptation and fetal capacity to increase O(2) extraction indicates that deficit in fetal oxygen delivery is unlikely to be causally associated with the altitude- and ancestry-related differences in fetal growth.

The fetal portal vein: normal blood flow development during the second half of human pregnancy

OBJECTIVES

The umbilical vein is the major source of blood for the fetal liver, but portal blood, which has not been measured directly in human fetuses, probably constitutes a physiologically important contribution. We aimed to establish a method for measuring blood flow in the fetal portal vein and to produce longitudinal reference ranges for diameter, blood flow velocities and volume blood flow during the second half of pregnancy.

METHODS

After a pilot project to develop the measuring technique, 160 low-risk pregnant women were recruited to a longitudinal study that included ultrasound examinations at 4-5-week intervals starting at 20-22 weeks of gestation. The inner diameter (D) of the vessel was measured with a perpendicular insonation angle, and the time-averaged maximum blood velocity (TAMXV) and weighted mean velocity (VWMEAN) were recorded with an insonation directed along the long axis of the vessel. Portal flow was calculated as: Q = pi(D/2)(2) x h x TAMXV (where h is a velocity profile parameter and equals 0.5). Portal flow was normalized for estimated fetal weight based on birth-weight percentiles.

RESULTS

The portal vein could be visualized and measured in 558/593 (94%) cases. Blood flow was pulsatile. D and TAMXV doubled during the second half of pregnancy (21-39 weeks), from 1.5 to 3.4 mm and from 8.4 to 14.9 cm/s, respectively. Correspondingly, the portal flow increased from 5 to 41 mL/min, and from 10 to 13 mL/min/kg when normalized for fetal weight. Similar results were achieved for VWMEAN.

CONCLUSION

Portal flow can be measured with a standardized technique. It increases during the second half of pregnancy, even when normalized for fetal weight, suggesting that it has increasing importance in the fetal liver circulation.

Longitudinal reference ranges for ductus venosus flow velocities and waveform indices

OBJECTIVES

Serial Doppler measurements of the ductus venosus are used increasingly for monitoring fetuses at risk of hemodynamic compromise, but existing reference ranges are based on cross-sectional studies and thus are less suitable for comparison with serial measurements. We aimed to establish longitudinal reference ranges for ductus venosus flow velocities and waveform indices and to provide the necessary terms for calculating conditional reference ranges for serial measurements.

METHODS

This was a longitudinal study of 160 low-risk pregnancies. Pulsed Doppler ultrasound was used to record ductus venosus blood flow velocities at 4-week intervals from 20-22 weeks of gestation onwards.

RESULTS

With a success rate of 93%, 547 measurements (four or five in each fetus) were used to establish reference ranges. The time-averaged maximum velocity was 50 cm/s at 21 weeks of gestation, increased to 60 cm/s at 32 weeks, and remained so until term. Similarly, the peak systolic velocity increased from 59 cm/s at 21 weeks to 71 cm/s at 31 weeks and remained so until term. The end-diastolic velocity showed a continuous increase from 31 cm/s at 21 weeks to 43 cm/s at 40 weeks. The pulsatility index for veins decreased from 0.57 at 21 weeks to 0.44 at 40 weeks. When conditioned by a previous measurement, the reference ranges for the next observation became narrower and commonly shifted compared with those of the entire population.

CONCLUSION

The new longitudinal reference ranges presented here reflect the development of the ductus venosus flow velocities and velocity indices and are thus appropriate for serial measurements, particularly if conditional terms are included.

Effect of umbilical ring constriction on Wharton's jelly

OBJECTIVE

The turgor of Wharton's jelly depends on osmotic and hydrostatic pressures. We tested the hypothesis that umbilical ring constriction has an impact on umbilical venous hemodynamics and thus on the volume of Wharton's jelly.

METHODS

In a cross-sectional study of 237 low-risk singleton pregnancies, the cross-sectional area of the fetal end of the umbilical cord was determined using sonography at 20-41 weeks of gestation. The inner area of the two arteries and the vein was also measured and subtracted from the cord area to calculate the area of Wharton's jelly. Based on the Bernoulli equation, the degree of vein constriction at the umbilical ring was assessed using the blood velocity increment at the abdominal inlet. Regression analysis and SD-score statistics were used to construct mean values and to assess the effects. The dataset was also analyzed for gender-specific effects.

RESULTS

The umbilical cord cross-sectional area increased with gestational age during the period 20-31 weeks, remaining essentially stable thereafter. The Wharton's jelly increased with gestational age from 20 until 31-32 weeks of gestation and remained at the same level for the rest of the pregnancy. At mid-gestation, on average 70% of the cord cross-sectional area was occupied by Wharton's jelly; at 31 weeks and later this value was 60%. Umbilical vein constriction was associated with reduced umbilical cord cross-sectional area and Wharton's jelly in female fetuses (P = 0.0007 and P = 0.003, respectively), but not in male fetuses.

CONCLUSIONS

Under physiological conditions, umbilical ring constriction affects umbilical vein hemodynamics, with corresponding effects on the umbilical cord cross-sectional area and the amount of Wharton's jelly. Interestingly, the effects are gender-specific.

Umbilical vein constriction at the umbilical ring: a longitudinal study

OBJECTIVE

It has been suggested that constriction of the umbilical vein (UV) at the umbilical ring has hemodynamic effects. We aimed to determine the occurrence and extent of such constriction in serial observations.

METHODS

This was a prospective longitudinal study of UV velocities at the umbilicus measured at approximately 4-week intervals between 19 and 42 weeks' gestation in 129 low-risk singleton pregnancies. Each participant was examined three to five times. Multilevel modeling was used to construct the reference ranges and to test associations between variables.

RESULTS

Gestational age-specific reference percentiles of UV velocities at the umbilicus were established based on 469 observations. Fetuses were able to alter the UV velocities considerably during the second half of pregnancy, signifying a varying degree of UV constriction. Of a total of 129 fetuses, 56 (43.4%) never had high UV blood velocity (i.e. > 46 cm/s, the highest quartile), 42 (32.6%) fetuses had high UV blood velocity on one occasion and 31 (24.0%) fetuses on two or more occasions. In 36 (27.9%) fetuses the UV velocity at the umbilical ring was > 300% of the mean gestational age-specific reference value at the intra-abdominal section on at least one occasion. Constriction of the UV at the umbilical ring did not affect the pulsatility of the umbilical artery, and was not associated with adverse perinatal outcome in this study.

CONCLUSIONS

Low-risk fetuses may well constrict the UV at the abdominal wall with velocities extending over wide ranges on one or more occasions during the second half of pregnancy. Rather than being a risk for complications, the constriction seems to be part of physiological development and possibly a regulatory mechanism.

The effect of umbilical venous constriction on placental development, cord length and perinatal outcome

BACKGROUND

Umbilical vein constriction at the fetal abdominal inlet is a common finding after week 13, when the period of umbilical herniation is brought to an end.

AIMS

To test the hypothesis that a constricting umbilical ring within physiological ranges affects fetal hemodynamics by either pooling blood in the placenta or restricting nutrient transfer to the fetus and thus shift the birthweight/placental weight (BW/PW) ratio. A constriction could also cause pressure changes and elongation of the cord and possibly be a disadvantage during labour.

STUDY DESIGN

Cross-sectional.

SUBJECTS

359 Low-risk singleton pregnancies at 13-40 weeks of gestation.

OUTCOME MEASURES

Standard deviation score (z-score) and regression analysis were used to determine the effect of umbilical vein constriction (expressed by increased blood velocity) on birthweight/placental weight ratio (BW/PW), cord length, Apgar score and emergency delivery due to fetal distress.

RESULTS

Umbilical venous constriction had a mild but significant effect on BW/PW in male (p=0.018) but not in female fetuses. Increased constriction was also associated with increased length of the cord but only in female fetuses (p=0.019). Cord length was positively related to birthweight and placental weight, but an increased length of the cord was also associated with decreasing BW/PW ratio for the male fetuses only (p=0.044). Increasing degree of venous constriction was associated with Apgar score < or =7 at 1 (p=0.009) but not at 5 min after birth and was not associated with emergency delivery.

CONCLUSION

Physiological umbilical venous constriction exerts a mild but significant gender-specific hemodynamic impact on intrauterine development.

Sonographic atypical vascular coiling of the umbilical cord

OBJECTIVE

To investigate whether an atypical umbilical coiling pattern at prenatal sonography is associated with adverse pregnancy outcome.

METHODS

A targeted sonographic evaluation of the umbilical cord (UC) was performed in 758 women with singleton gestation, and gestational age above 20 weeks. Atypical coiling was defined as the presence of a spring-shape UC (supercoiling) or an unusual, aperiodic coiling pattern (uncoordinated coiling). Umbilical artery Doppler assessment was conducted in cases with atypical coiling. Pregnancy and neonatal outcomes were investigated.

RESULTS

Of the study population, 7 and 16 fetuses had an umbilical cord with uncoordinated coiling and supercoiling respectively. Three umbilical cords had a single umbilical artery. Eight patients delivered before 34 weeks of gestation. Eight fetuses were growth restricted. In seven cases, abnormal sonographic findings were detected (three meconium peritonitis, two severe hydronephrosis and two cardiac anomalies). One fetus affected by trisomy 18 presented multiple anomalies. Perinatal death occurred in three cases. Of the surviving newborns, eight were admitted to NICU. Umbilical artery Doppler waveforms presented a systolic notch in seven (30.4%) cases.

CONCLUSIONS

The presence of an atypical umbilical cord vascular coiling is associated with an increased risk of unfavourable pregnancy outcome. The identification of an umbilical artery notch at Doppler investigation is frequently associated with an atypical UC coiling pattern.

Reference ranges for umbilical vein blood flow in the second half of pregnancy based on longitudinal data

OBJECTIVES

To construct new reference ranges for serial measurements of umbilical vein (UV) blood flow.

METHODS

Prospective longitudinal study of blood flow velocities and diameter of the UV measured at four-weekly intervals during 19 to 42 weeks' gestation in 130 low-risk singleton pregnancies. Regression models and multilevel modeling were used to construct the reference ranges.

RESULTS

On the basis of 511 sets of longitudinal observations, we established new reference percentiles of UV diameter, blood flow velocities, volume flow, and blood flow normalized for fetal weight and abdominal circumference. They reflected some of the developmental patterns of previous cross-sectional studies, but with important differences, particularly near term. The UV blood flow showed a continuous increase until term, whereas the flow normalized per unit fetal weight, a corresponding reduction. Calculating the blood flow on the basis of intensity-weighted mean velocity or 0.5 of the maximum velocity gave almost interchangeable results for most fetuses.

CONCLUSION

New reference ranges for UV blood flow based on longitudinal observations appear slightly different from cross-sectional studies, and should be more appropriate for serial evaluation of fetal circulation.

The effect of vascular constriction on umbilical venous pulsation

OBJECTIVES

Umbilical venous pulsation is an important sign of hemodynamic compromise, but is also found under normal physiological conditions. Mathematical modeling suggests that vascular compliance is a determinant for pulsation, and we tested this by studying velocity pulsation at three sites on the umbilical vein.

METHODS

In a cross-sectional study of 279 low-risk pregnancies (20-40 weeks' gestational age) blood flow velocity in the umbilical vein was determined before, within and after the umbilical ring in the fetal abdominal wall, and the incidence and magnitude of pulsation (the difference between the maximum and minimum velocity during a pulse, and pulsatility index) were noted. Based on the fact that the vessel cross-sectional area is an important determinant of compliance, we measured the diameter and time-averaged maximum velocity to reflect variation in diameter and compliance at the three sites.

RESULTS

The incidence of umbilical venous pulsation was higher at the umbilical ring in the abdominal wall (242/279, 87%, 95% CI 82-90) than in the cord (43/198, 22%, 95%CI 16-27) or intra-abdominally (84/277, 30%, 95% CI 25-36) (P < 0.001). When pulsation was observed intra-abdominally, the pulsatility was not different from that at the umbilical ring (P = 0.16). However, the lowest pulsatility was found in the cord vein (P < 0.0001), where the largest vein diameter was found.

CONCLUSION

The high incidence of venous pulsation at the umbilical ring where diameter and compliance are low supports the suggestion that local compliance is an important factor influencing pulsation in fetal veins.

The fetal circulation

Accumulating data on the human fetal circulation shows the similarity to the experimental animal physiology, but with important differences. The human fetus seems to circulate less blood through the placenta, shunt less through the ductus venosus and foramen ovale, but direct more blood through the lungs than the fetal sheep. However, there are substantial individual variations and the pattern changes with gestational age. The normalised umbilical blood flow decreases with gestational age, and, at 28 to 32 weeks, a new level of development seems to be reached. At this stage, the shunting through the ductus venosus and the foramen ovale reaches a minimum, and the flow through the lungs a maximum. The ductus venosus and foramen ovale are functionally closely related and represent an important distributional unit for the venous return. The left portal branch represents a venous watershed, and, similarly, the isthmus aorta an arterial watershed. Thus, the fetal central circulation is a very flexible and adaptive circulatory system. The responses to increased afterload, hypoxaemia and acidaemia in the human fetus are equivalent to those found in animal studies: increased ductus venosus and foramen ovale shunting, increased impedance in the lungs, reduced impedance in the brain, increasingly reversed flow in the aortic isthmus and a more prominent coronary blood flow.