[Significance of fetal factors in prognosis of outcome after labor induction]

Aim of the present mixed prospective and retrospective study is to determine the prognostic significance of fetal factors for the continuity and the final outlet of induction of labor. Factors like fetal weight, placental maturity according Grannum, status of the amniotic membranes and amniotic fluid index by Phelan were examined. Based on the given results the authors conclude that there is no dependence between the expected overweight of the fetus and the continuity and the final outcome of induction of labor. The immature placenta and the increased amniotic fluid index are related with prolonged inductions of labor but the terminal outcome is not influenced by them. Premature rupture of the amniotic membranes cause prolonged inductions and the preserved membranes with an increased frequency of unsuccessful inductions.

Vasopressin receptor expression in the placenta

The arginine vasopressin (AVP) type 1a receptor (V1a) is well known to mediate vasoconstriction. In pregnancy, blood flow in the placenta is crucial for sustaining normal growth and development of the fetus. This is the first AVP receptor study in the placenta and fetal membranes. The aim was to compare, quantitatively, the level of V1a gene expression with that of a known marker for vascularization, aquaporin 1 (AQP1). V1a and AQP1 gene expression did not correlate; placental V1a mRNA levels were significantly upregulated at 45 and 66+/-1 compared with 27, 100+/-4, and 140 days (term approximately 150 days). V1a mRNA levels were much lower in fetal membranes in which no significant difference across gestation was observed. In situ hybridization histochemistry localized V1a gene expression in the maternal component of the placenta similar to the receptor-binding studies using 125I-labeled [d(CH2)5, sarcosine7] vasopressin. No AVP gene expression was observed in the placenta and fetal membranes, which eliminates local AVP production. This increase in V1a expression at 45 and 66+/-1 days of gestation correlates with the period of maximal placental growth in the sheep and suggests that AVP and V1a receptors may play a hitherto unrecognized role in placental growth, differentiation, and/or function, particularly in the deleterious effects of heat stress, early in pregnancy, on fetal growth.

Morphology and mucosal biochemistry of gastroschisis intestine in urine-free amniotic fluid

BACKGROUND/PURPOSE

The aim of this study was to determine the role of urine exposure in gastroschisis on the pathologic and biochemical aspects.

METHODS

The intestines of fetal rabbits with gastroschisis (group G), gastroschisis and urethral ligation (group GL), and normal controls (group C) were studied by measuring weigh and length, intestinal diameter and wall thickness, and thickness of each intestinal layer. Number and length of villi and villi edema were evaluated. Total protein and DNA were measured in intestinal homogenate. Lactase activity and alkaline phosphatase activity were analyzed in isolated microvilli membranes.

RESULTS

Intestinal length, diameter, and wall thickness were significantly different in groups G and GL compared with C but not between groups G and GL. The same was true for the thickness of the internal muscular and serosa, villi length, and villi edema. Serosal reaction was milder in group GL than in group G, and absent in C. Total protein, lactase activity, and alkaline phosphatase activity were also significantly different in G and GL compared with C but not between G and GL.

CONCLUSIONS

Urine in amniotic fluid causes an increased serosal reaction but does not account for the mucosal enzyme dysfunction.

Effect of amniotic fluid upon prostaglandin E2 and I2 production by cultured human myometrial cells

OBJECTIVE

Our goal was to study the effect of amniotic fluid obtained at 16 and 39 weeks of gestation in normal human pregnancies upon prostaglandin production by human myometrial cells in culture.

STUDY DESIGN

Amniotic fluid, sampled either at 16 weeks, during amniocentesis, or at 39 weeks, during caesarean section before labor, was fractionated by molecular-weight and then incubated with human myometrial cells in culture. We then used radioimmunoassay to measure PGE(2) and PGI(2) production.

RESULTS

The "3-30 kDa" fraction of amniotic fluid sampled at 16 weeks significantly inhibited PGE(2) and PGI(2) production by human myometrial cells. When amniotic fluid was sampled at 39 weeks, it stimulated both PGE(2) and PGI(2) production, and the ">30 kDa" fraction increased levels of PGE(2) considerably more than of PGI(2) (420.0+/-88.0 ng/10(6)cells versus 188.2+/-21.4 ng/10(6)cells, P<0.001).

CONCLUSION

Amniotic fluid contains substances whose effects in cultured myometrial cells vary according to gestational age and type of prostaglandin. These data suggest that the fetus plays a role in the regulation of myometrial activity during pregnancy.

Predictability of intrapartum and neonatal outcomes with the amniotic fluid volume distribution: a reassessment using the amniotic fluid index, single deepest pocket, and a dye-determined amniotic fluid volume

OBJECTIVE

The purpose of this study was to establish whether ultrasound-estimated or dye-determined amniotic fluid distribution (upper compared with lower quadrant) is predictive of perinatal outcome.

STUDY DESIGN

Amniotic fluid distribution as measured by the amniotic fluid index, single deepest pocket, and dye-determined volumes was ascertained and correlated with intrapartum and neonatal outcomes.

RESULTS

Between January 1997 and January 2001, 135 women (70 upper-greater and 65 lower-greater) participated in this prospective observational study. The sum of the amniotic fluid index (P =.309), single deepest pocket (P =.168), and dye-determined amniotic fluid volume (P =.368) for the upper-greater compared with the lower-greater groups were similar. Decelerations in labor (P =.597), late decelerations (P =.999), cesarean deliveries for fetal distress (P =.413), and umbilical cord pH < 7.2 were similar (P =.647)

CONCLUSION

Ultrasound-estimated and dye-determined amniotic fluid volumes are similar between upper-greater and lower-greater groups, and intrapartum/neonatal outcomes are not affected by the amniotic fluid distribution.

Fetal responses to maternal and intra-amniotic lipopolysaccharide administration in sheep

A link between intrauterine infection and premature labor is widely accepted, yet the fetal inflammatory responses to such infections are not well understood. Our aim was to use a sheep model in which an inflammatory state was induced by lipopolysaccharide (LPS) administration during pregnancy to the maternal systemic, intra-amniotic or extra-amniotic compartments. Fetal and maternal blood gases and uterine electromyographic activity along with fetal and maternal circulating concentrations of prostaglandins PGE2 and PGFM, cortisol, and interleukin-6 were determined. Maternal systemic LPS treatment resulted in mild maternal hypoxemia, a rise in temperature, greater fetal hypoxemia, and a marked rise in fetal cortisol and PGE2 concentrations that persisted for 48 h. Intra-amniotic administration of LPS at doses higher than those used systemically caused an increase in fetal cortisol and PGE2 concentrations as well as a rise in uterine activity, but these were lesser in magnitude. Extra-amniotic LPS administration caused no overt fetal or maternal inflammatory responses. We conclude that maternal LPS treatment markedly elevated fetal cortisol and PGE2 concentrations. This may be a potential protective mechanism that aids the fetus in the event of premature delivery. The attenuated fetal response to intra-amniotic LPS treatment, despite the much higher dose used, may support a role for the amniotic fluid in protecting the fetus from endotoxin exposure during pregnancy.

Prolonged culture of normal chorionic villus cells yields ICF syndrome-like chromatin decondensation and rearrangements

Untreated cultures from normal chorionic villus (CV) or amniotic fluid-derived (AF) samples displayed dramatic cell passage-dependent increases in aberrations in the juxtacentromeric heterochromatin of chromosomes 1 or 16 (1qh or 16qh). They showed negligible levels of chromosomal aberrations in primary culture and no other consistent chromosomal abnormality at any passage. By passage 8 or 9, 82 +/- 7% of the CV metaphases from all eight studied samples exhibited 1qh or 16qh decondensation and 25 +/- 16% had rearrangements in these regions. All six analyzed late-passage AF cultures displayed this regional decondensation and recombination in 54 +/- 16 and 3 +/- 3% of the metaphases, respectively. Late-passage skin fibroblasts did not show these aberrations. The chromosomal anomalies resembled those diagnostic for the ICF syndrome (immunodeficiency, centromeric region instability, and facial anomalies). ICF patients have constitutive hypomethylation at satellite 2 DNA (Sat2) in 1qh and 16qh, generally as the result of mutations in the DNA methyltransferase gene DNMT3B. At early and late passages, CV DNA was hypomethylated and AF DNA was hypermethylated both globally and at Sat2. DNMT1, DNMT3A, or DNMT3B RNA levels did not differ significantly between CV and AF cultures or late and early passages. The high degree of methylation of Sat2 in late-passage AF cells indicates that hypomethylation of this repeat is not necessary for 1qh decondensation. Sat2 hypomethylation may nonetheless favor 1qh and 16qh anomalies because CV cultures, with their Sat2 hypomethylation, displayed 1qh and 16qh decondensation and rearrangements at significantly lower passage numbers than did AF cultures. Also, CV cultures had much higher ratios of ICF-like rearrangements to heterochromatin decondensation in chromosomes 1 and 16. These cultures may serve as models to help elucidate the biological consequences of cancer-associated satellite DNA hypomethylation.

A randomized trial using ultrasound to identify the high-risk fetus in a low-risk population

OBJECTIVE

To evaluate the effect of introducing two biophysical ultrasound examinations in a low-risk antenatal population. Scans were performed at 30-32 weeks' gestation and 36-37 weeks' gestation.

METHODS

Scans assessed placental maturity, amniotic fluid volume, and estimated fetal weight. One thousand nine hundred ninety-eight low-risk patients were randomized at 30 weeks' gestation to a control group receiving standard antenatal care, or to the study group who also received an ultrasound scan. Outcome measures were frequency of small for dates (less than 10th percentile at birth), intervention rates, and admissions to neonatal intensive care.

RESULTS

The proportion of infants assessed as small for dates at birth in the study group was 6.9% (69 of 994) compared with 10.4% (104 of 999) in the control group (P =.008). The rates of intervention in the study and control groups were 31.3% (313 of 999) and 16.9% (169 of 999), respectively (P <.001). Twenty-eight (2.8%) neonates in the study group were admitted to the neonatal unit compared with 34 (3.4%) in the control group (P =.532).

CONCLUSION

Introduction of an ultrasound scan at 30-32 weeks' and 36-37 weeks' gestation may reduce the risk of a growth-restricted infant and increases antenatal interventions. Rates of admission to a neonatal unit are not significantly affected.

Reversal of pregnancy-mediated plasma hypotonicity in the near-term rat

OBJECTIVE

Maternal plasma hypotonicity occurs early in rat and human pregnancy with resetting of the plasma osmolality threshold for vasopressin secretion and thirst. In humans, amniotic fluid volume reaches maximum levels in the mid-third trimester and decreases thereafter. We hypothesized that a reversal of maternal plasma hypotonicity occurs near term, contributing to reduced fetal and amniotic fluid water content.

METHODS

Maternal plasma and amniotic fluid osmolality and sodium levels, including amniotic fluid volume, were measured at 16, 18 and 20 days of rat gestation. Additionally, maternal and fetal brains were analyzed for water and electrolyte content. Non-pregnant adult female rats represented controls.

RESULTS

Compared to non-pregnant adults, 16-day and 18-day pregnant rats had significantly lower plasma osmolality (301.0 +/- 2.3 vs. 295.4 +/- 2.8 and 289.7 +/- 3.3 mOsm/kg, respectively) and sodium levels (140.3 +/- 1.0 vs. 135.7 +/- 0.8 and 133.4 +/- 1.4 mEq/l, respectively). Conversely, 20-day pregnant rats showed no significant difference in plasma osmolality (298.4 +/- 3.1 mOsm/kg) or sodium levels (137.6 +/- 1.0 mEq/l) from non-pregnant adults. With advancing gestation, the amniotic fluid volume decreased whereas the osmolality and sodium levels increased significantly. Maternal brain water content was significantly higher in 16-day and 18-day pregnant rats compared to control rats (78.7 +/- 0.1 and 78.1 +/- 0.2 vs. 76.9 +/- 0.2% wet weight) and returned to non-pregnant values in the 20-day pregnant rats (76.6 +/- 0.2%). In association with the maternal changes, fetal brain water and electrolyte content significantly decreased from 16-day to 18-day to 20-day fetuses.

CONCLUSION

These findings indicate a reversal of maternal plasma hypotonicity and reduced maternal brain water content in the near-term pregnant rat. We speculate that relative maternal plasma hypertonicity near term may contribute to reduced amniotic fluid volume.

Effects of human amniotic fluid on peripheral nerve scarring and regeneration in rats

OBJECT

Peripheral nerve repair surgery is still replete with challenges. Despite technical improvements in microsurgery, classic methods of nerve repair have failed to provide satisfactory results. The purpose of this study was to investigate the effects of amniotic fluid from humans on peripheral nerve scarring and regeneration in rats.

METHODS

Forty adult Sprague-Dawley rats were used in this study. After the right sciatic nerve in each rat was transected and repaired using an epineural suture procedure, the nerves were divided into two groups according to the solution applied around the repair site: experimental group, 0.3 ml human amniotic fluid (HAF); and control group, 0.3 ml saline. Macroscopic and histological evaluations of peripheral nerve scarring were performed 4 weeks postsurgery. Nerves treated with HAF demonstrated a significant reduction in the amount of scar tissue surrounding the repair site (p < 0.05). No evidence of a reaction against HAF was noted. Functional nerve regeneration was measured once every 2 weeks by using a sciatic function index until 12 weeks postsurgery. Functional recovery in nerves treated with amniotic fluid occurred significantly faster than that in nerves treated with saline (p < 0.05). Peripheral nerve regeneration was evaluated histomorphologically at 12 weeks postsurgery. Nerves treated with amniotic fluid showed significant improvement with respect to the indices of fiber maturation (p < 0.05).

CONCLUSIONS

Preliminary data show that HAF enhances peripheral nerve regeneration. The preventive effect of HAF on epineural scarring and the rich content of neurotrophic and neurite-promoting factors possibly contribute to this result.

Fractalkine (FRK) levels in amniotic fluid and its production during pregnancy

Fractalkine is a new CX(3)C chemokine that has chemoattractant activity for T cells, monocytes and natural killer (NK) cells. Western blot analysis revealed that fractalkine protein was detected as a 95 kDa band in both the amniotic fluid and the amnion during the second and third trimesters. Immunohistochemistry using an anti-fractalkine polyclonal antibody revealed positive staining of epithelial cells in amnion and trophoblasts in both the second and third trimesters. Neonatal urine also contained detectable amounts of fractalkine. RT-PCR detected fractalkine mRNA transcripts in the amnion. To determine whether fractalkine receptor (CX(3)CR1)-positive cells were present in amniotic fluid and amnion, we performed RT-PCR using specific primers for CX(3)CR1. CX(3)CR1-positive cells had migrated into the amniotic fluid and the amnion. The present findings suggest that fractalkine found in amniotic fluid may contribute to the immunodefence mechanism during pregnancy.

Deepest vertical amniotic fluid pocket at term. Normal values and clinical application

OBJECTIVE

To determine the normal values of the deepest vertical amniotic fluid pocket (DVP) in term pregnancies and whether a similar relationship to birth weight (BW) exists.

STUDY DESIGN

Two hundred thirty-one term patients (37-42 weeks) with intact membranes and a normal amniotic fluid index (AFI) were included. DVP was identified and compared to BW, BW > 4,000 g (large for gestational age [LGA]) and BW < 2,500 g (small for gestational age [SGA]). Statistical comparisons and linear regression models were made.

RESULTS

The mean gestational age was 39.6 weeks, and mean BW 3,284 g. Mean DVP was 4.5 cm. This did not vary significantly by gestational age. A positive linear correlation was observed between DVP and BW. DVP was greater for pregnancies with LGA (5.2 vs. 4.3 cm [P < .003]). No difference was observed for SGA. The relative risk for a fetus > 4,000 g when the DVP was > 6 was 4.0 (1.6-9.5) and 15.8 (1.6-157.6) if maternal diabetes was also present. No difference was determined for SGA using a DVP < 2.

CONCLUSION

Higher DVP is associated with delivery of an LGA infant, particularly in diabetic women.

The amniotic fluid index in late pregnancy

OBJECTIVE

Our purpose was to examine changes in the amniotic fluid index (AFI) in accurately dated term pregnancies both in relation to gestational age and in relation to the onset of spontaneous labor.

STUDY DESIGN

This was a prospective observational study in 137 women with uneventful term pregnancies, in whom 220 AFI measurements were performed. More than one AFI value was available from 51 individuals.

RESULTS

The AFI did not change significantly between 37 and 42 weeks' gestation, but a significant reduction was seen during the last 11 days before the spontaneous onset of labor (R = -0.37, n = 83, p < 0.001). The AFI (corrected for gestational age) within individuals remained stable over periods of up to 2 weeks. Meconium staining of the amniotic fluid was related to gestational age, but not to the AFI or fetal distress at birth. No significant correlation was found between fetal distress and the AFI, or between fetal distress and the reduction in AFI during the last two measurements before labor.

CONCLUSIONS

The reduction of the AFI in pregnancies progressing beyond term is related to the labor process itself rather than to the exact gestational age.

Microscopic changes induced by the intratracheal inoculation of amniotic fluid and meconium in the lung of neonatal rats

Meconium aspiration syndrome is a major contributor to neonatal respiratory distress in infants and it has been sporadically recognized in neonatal animals. This investigation was designed to study the short and long term effects of meconium and amniotic fluid in the lungs of neonatal rats. Seven-day-old rats (n = 123) divided in three groups were intratracheally inoculated with saline solution, amniotic fluid or meconium. Rats were euthanatized on 1, 3, 7, 14, 28, 56 and 112 postinoculation days (PID) and the lungs were examined by light microscopy. Saline solution did not induce any change while amniotic fluid elicited only a mild foreign body response which disappeared by PID 14. In contrast, meconium induced an exudative alveolitis characterized by recruitment of neutrophilsn in the bronchoalveolar spaces. Meconium also induced atelectasis, hyperinflation and thickening of alveolar septa all of which had disappeared by PID 14. Starting at PID 7, neutrophils were progressively replaced by macrophages, giant cells, and some fibroblasts. There were sporadic foci of mineralization starting at PID 14 and lasting up to PID 112. Some mineralized foci became lined with cuboidal epithelial cells at PID 28. Meconium was slowly degraded but still evident by PID 112. It was concluded that inoculation of meconium in neonatal rats induces acute microscopic changes typical of meconium aspiration syndrome. The long term lesions induced by meconium consisted of persistent multifocal histiocytic alveolitis and bronchiolitis reaction with occasional foci of calcification.

[Morphologic study of the intestine in an experimental model of amnioinfusion in fetal rabbits with gastroschisis]

An experimental model of serial amnioinfusion has been developed in fetal rabbits with gastroschisis, using an intraamniotic catheter connected to a subcutaneous port. Fetuses of 4 groups were compared 7 days after surgery: group A: gastroschisis and daily amnioinfusion through an implanted catheter; group C: gastroschisis and blind amniotic catheter; group G: gastroschisis without catheter; group O: nonoperated fetuses. Survival rate, fetal body weight, lung weight, intestinal weight and length were determined. Computer aided morphometric analysis was performed, in which intestinal diameter, thickness and villi length were measured. Amniotic fluid samples were recovered along the experimental period. Intestinal length was significantly shorter and had a significantly thicker wall than nonoperated fetuses; we found no other morphometric differences between gastroschisis treated with amnioinfusion (group A) and the other gastroschisis groups (C and G). Amnioinfusion did not affect fetal survival rate; the amniotic catheter alone did not cause pulmonary hypoplasia due to significant amniotic leak. The physiological decrease in amniotic volume towards the end of gestation has not been modified by this regime of amnioinfusion.

Cytokine concentrations in the amniotic fluid during parturition at term: correlation to lower uterine segment values and to labor

BACKGROUND

In previous studies a significant increase in interleukin (IL)-6 and IL-8 concentrations in the lower uterine segment parallel to cervical dilatation at term could be found, however only a weak correlation to duration of labor was detected. This study compares amniotic fluid concentrations of interleukin (IL)-6 and IL-8 with those in the lower uterine segment, and the duration of labor.

METHODS

Amniotic fluid and lower uterine segment specimens were obtained from 29 patients during cesarean section at term. The patients were divided into groups according to cervical dilatation (< 2 cm, 2-3.9 cm, 4-6 cm, > 6 cm) and to labor (0 h, > 0-12 h, > 12 h). Interleukin-6 and IL-8 concentrations were determined by enzyme immunoassay.

RESULTS

Amniotic fluid IL-6 already increased at 2-3.9 cm (p = 0.02), while the steepest increase in IL-8 was observed at > 6 cm (p = 0.003). No correlation with lower uterine segment values was observed for either of the cytokines. However, the amniotic fluid IL-6 concentration correlated with labor (p = 0.0008).

CONCLUSION

The increase in the concentration of IL-6 in the amniotic fluid earlier than its increase in the lower uterine segment supports the hypothesis that IL-6 plays a crucial role for the promotion of labor in the first place, and afterwards for the biochemical degradation processes in the lower uterine segment. The fact that the greatest increase in IL-8 concentration occurs only at > 6 cm indicates that this chemotactic cytokine has only minor significance in the initiation of parturition by its concentration in the amniotic fluid.

Substance abuse and polyhydramnios

OBJECTIVE

Maternal drug use is known to have fetal central nervous system depressive effects as manifested by decreased biophysical profile scores. Drug effects on the amniotic fluid index (AFI), one variable of the biophysical profile, are not well documented. Our objective was to determine the effects of maternal drug use on the AFI.

STUDY DESIGN

Over an 18-month period from July 1997 to December 1998, consecutive cases of women with toxicology-proven drug use during pregnancy were identified. AFI values and the rate of polyhydramnios in substance users were compared with those observed at our institution in more than 6400 patients undergoing ultrasound testing during the same period as the cases.

RESULTS

Sixty-three toxicology-positive cases were identified. Although there was no significant difference between mean AFI values, the incidence of polyhydramnios (AFI >24 cm) was significantly higher in substance users (28.6%) than in control patients (3.9%) (P <.005).

CONCLUSION

Maternal substance abuse is associated with a significantly higher incidence of polyhydramnios and should be considered a possible etiologic factor in women with apparent idiopathic polyhydramnios.

Effect of maternal oral hydration on amniotic fluid index in women with pregnancy-induced hypertension

OBJECTIVE

To evaluate the effect of maternal oral hydration on amniotic fluid index (AFI) in women with pregnancy-induced hypertension.

METHODS

Five women with pregnancy-induced hypertension and five normotensive gravidae at 32-37 weeks gestation with normal AFI (8-18cm) were hydrated by oral intake of 2L of water over 1 h. Both groups were similar in age, parity and gestation. Repeat AFI was measured in all women after 3 h. The differences between pre- and post-hydration AFI in the two groups were analyzed with Student's t-test for statistical significance.

RESULTS

Maternal oral hydration resulted in a significant increase in AFI in both hypertensive and normotensive gravidae (1.7cm and 3.48cm, respectively). However, the extent of increase was significantly lower in the hypertensive group (P = 0.03).

CONCLUSION

Pregnancy-induced hypertension adversely affects the increase in amniotic fluid volume with maternal hydration. This observation is of clinical relevance in the management of oligohydramnios associated with pregnancy-induced hypertension.

A review of amniotic fluid dynamics and the enigma of isolated oligohydramnios

Traditionally, oligohydramnios has been implemented as a sign of potential fetal compromise and associated with an increased incidence of adverse perinatal morbidity and mortality. Decreased amniotic fluid volume is especially of concern when it occurs in conjunction with structural fetal anomalies, fetal growth restriction, postdates pregnancies, and maternal disease. Consequently, following ultrasonographic diagnosis of oligohydramnios at term, delivery is routinely advocated even in otherwise uncomplicated pregnancies with an appropriate-for-gestational-age fetus, irrespective of the presence of reassuring fetal evaluation and the absence of maternal disease. Numerous factors complicate the ultrasonographic diagnosis of oligohydramnios. These include a lack of complete detailed understanding of the physiology of the dynamics of oligohydramnios, the transient condition at times of decreased amniotic fluid volume, generally poor performance of ultrasonography in detecting oligohydramnios, an array of different ultrasound diagnostic criteria, and varying ultrasonographic thresholds. In light of the latter and the lack of prospective randomized data, is unclear that the practice of effecting delivery for isolated oligohydramnios at term is justified. This article presents physiologic dynamics of amniotic fluid, factors that may affect amniotic fluid volume, possible pitfalls in the ultrasonographic assessment of amniotic fluid volume, and the clinical significance of oligohydramnios. In addition, the literature regarding perinatal outcome associated with oligohydramnios and current available data supporting expectant noninterventional management of cases complicated by isolated oligohydramnios at term are discussed.

Does amniotic fluid volume affect fetofetal transfusion in monochorionic twin pregnancies? Modelling two possible mechanisms

Clinical evidence suggests that increased amniotic fluid volume due to polyhydramnios increases placental vascular resistance. We have sought to model the possible effects of an increased amniotic fluid volume on the net fetofetal transfusion in monochorionic twin pregnancies. We wanted to compare these effects with the results of previous simulations, which aimed to explain why the twin-twin transfusion syndrome (TTTS) placentas more often include bidirectional arteriovenous (AV) rather than AV plus arterioarterial (AA) anastomoses. We extended our mathematical model of TTTS by simulating two different mechanisms that increase the placental vascular resistance as a consequence of polyhydramnios. First, there is an increase in the placental capillary resistance and hence in deep AV and opposite AV (denoted as VA) resistances due to polyhydramnios. Second, there is an increase in the resistance of chorionic veins due to polyhydramnios, assuming that these veins act as Starling resistors. We then simulated the effects of polyhydramnios on different placental anastomotic patterns. The results were as follows. In the first mechanism (polyhydramnios affects AV-VA resistances), an increased amniotic fluid volume hardly affected bidirectional AV, but slightly decreased fetofetal transfusion in AV plus AA anastomoses. However, for these effects to change the natural development of the pregnancy, polyhydramnios needed to persist for approximately 4 weeks, and by comparing the effects of polyhydramnios with the effects of amnioreduction, amnioreduction was more beneficial for normalizing the donor amniotic fluid volume. Therefore, these beneficial effects due to polyhydramnios have no practical clinical significance. In the second mechanism (Starling resistor for chorionic veins), polyhydramnios slightly increased fetofetal transfusion and hence slightly increased TTTS severity in bidirectional AV and AV plus VV, but did not affect AV plus AA anastomoses. In conclusion, we hypothesize that the simulated effects of polyhydramnios are not the primary cause of the fact that TTTS placentas more often include bidirectional AV than AV plus AA anastomoses. Rather, the more likely explanation is the previously identified larger range of AA than VA anastomotic diameters that adequately compensate for the effects of the AV.