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2009-12-08-14 Sirenomelia, first trimester © Solovyov

Sirenomelia, first trimester 

Oleksiy Solovyov, MD; Yana O. Goncharova, MD, PhD; Yuliya V. Masliy, MD, PhD; Prof. Iryna O. Sudoma, MD.

Clinic of reproductive medicine “NADIYA”, Kyiv, Ukraine;
Andrijivskyj uzviz, 28A, Kyiv 01025, Ukraine.


Mermaid syndrome; sirenomelia sequence.


The sirenomelia sequence is a rare and lethal congenital anomaly characterized by fusion, rotation, hypotrophy or atrophy of the lower limbs and severe urogenital abnormalities (usually associated with the absence of bladder and agenesia or dysgenesia of kidneys) leading to oligohydramnios in the second half of pregnancy. 

Case report

The case of sirenomelia was diagnosed during the routine first-trimester combined screening (OSCAR method - One Stop Clinic for Assessment of Risk of fetal anomalies). The patient was a 31-year-old primigravida after IVF (in vitro fertilization) due to her primary infertility. This was her first IVF/ ICSI (intracytoplasmatic sperm injection) attempt. Pregnancy was uncomplicated. The woman had normal karyotype herself and her husband’s karyotype was 46,XY, inv. 9 (p21q22) (normal variant). Family and personal history was unremarkable.

Our ultrasound examination was performed at 12 weeks of gestation by accredited sonologist, following the Fetal Medicine Foundation criteria. Nuchal translucency measured 1.3 mm.

Following findings were revealed:

  • inadequate size of the gestational sac, corresponding to 10 weeks of gestation

  • dilated urinary bladder

  • marked bilateral hydronephrosis

  • single umbilical artery

  • fused lower extremities resembling the mermaid"s tail.

Images 1,2: 12 weeks of pregnancy. Image 1 shows inadequate gestational sac size - the average size of the gestational sac was just 50 mm, corresponding to 10 weeks of pregnancy. The image 2 shows dilated urinary bladder of the fetus measuring 85x10x11.5 mm.


Images 3,4: Image 3 shows marked bilateral fetal hydronephrosis. Image 4 shows single umbilical artery running around the urinary bladder, depicted by power Doppler (left part of the image).


Image 5: Image shows fused lower extremities of the fetus resembling a mermaid"s tail.


Images 6, 7: 3D images show fused lower fetal extremities (arrows on the image 7 point at the fused legs of the fetus).


Sirenomelia was diagnosed and parents opted for the pregnancy termination due to the lethal prognosis for the fetus. Before the termination a transcervical fetoscopy was done, confirming the diagnosis (Images 8, 9, and 10). The fetoscopy was performed by STORZ diagnostic microhysteroscope with external size of 4 mm without cervical dilatation. Isotonic saline solution was used to keep the uterine cavity dilated. Initial flow of the saline solution was set to 100 ml/min, and after incision of the gestational sac the flow was reduced to 30-80 ml/min, which was sufficient to scrutinize the fetus and reduce the fetal movements caused by the liquid flow.

Images 8, 9: Fetoscopic images showing the fused lower fetal limbs.


Image 10: Image shows normal fetal head, hands and two-vessel umbilical cord.

The pregnancy was terminated by cervical dilation followed by an aspiration and curettage. Cytogenetic analysis of the removed material was done, revealing normal fetal female karyotype and mosaic karyotype of chorionic villi (normal female karyotype; near tetraploid karyotype, mos 46, XX [67 %] /~ 4n [33 %]).

Image 11: Fetal karyotype.

Combination of ultrasound examination, fetoscopy, and cytogenetic analysis enabled us to confirm initial diagnosis of the mermaid syndrome. Diagnosis of such an anomaly in the first trimester reduces the psychological trauma of the affected couples. Especially compare to revealing such a diagnosis later in the pregnancy or even after birth. The parents were informed about the population risk and prognosis for the further pregnancy. They have decided for another pregnancy via IVF technique.


The term sirenomelia comes from the greek words seiren = mermaid and melos = limb. The names “sirenomelia” or “mermaid syndrome” originate from the physical similarity of the affected fetuses to the mythical creature, mermaid. Mermaid was a magic women with lower part of the body in the form of fish tail. Fish tail is resembled by the fusion of the lower fetal limbs and partial or full fusion of the feet.

The table 1 summarizes all 10 cases of mermaid syndrome diagnosed in the first trimester found in the world medical literature (we used Google Internet searching tool, using Latin and Cyrillic letters; separate search among Polish, Ukrainian and Russian-speaking parts of Network did not give additional results).

Table 1: Cases of antenatal diagnosis of mermaid syndrome in the first trimester (Google Internet).


 Year of publication

Number in reference list

 Carbillon L et al.



 Monteagudo A et al.

 1st case 2002


 Monteagudo A et al.

 2nd case 2002


 Schiesser M et al.



 Van Keirsbilck J et al.



 Batukan C et al.



 Heflin D.  



 Trevisan R.



 Sebastião M et al.



 Akbayir O et al.



 Our current case




According to different authors the incidence of the sirenomelia varies from 0.1:10.000 to 0.47:10.000 live births [32,44,46,48,49,50,56]. Nevertheless, these data can be misleading, considering similarity of this defect to caudal dysplasia or regression.


Etiology of sirenomelia is unknown. There are some theories, but none of them is convincing enough [26,50,51,52,56,61]. There are some assumptions that sirenomelia is a consequence of teratogenic factors. Sirenomelia has occurred in animals after cadmium [23], retinoid acid (acid of vitamin A) [60], cocaine [45] or irradiation [49] exposure. Association of the sirenomelia with assisted reproductive technologies, namely with ICSI (Intracytoplasmic Sperm Injection) was also described [4].


Three pathogenetic theories of sirenomelia are described:

  • theory of pressure

  • theory of primary damage of caudal somites development that leads to the abnormal development of fetal lower part

  • theory of undernourishment of fetal caudal part.

For better understanding of the pathogenesis, a vascular stealing phenomenon was proposed [53]. Sirenomelia is probably a consequence of an abnormal blood supply of caudal parts of the embryo. A diverged abnormal vessel, being a derivate of yolk artery, deviates from abdominal aorta of affected fetuses and works as a single large umbilical artery (more often the right one). The vessel carries blood through the umbilical cord into the placenta. The fetus with sirenomelia has only two vessels (instead of three) running within the umbilical cord. The vessel "steals" blood from the structures located below its origin - from the tissues of fetal caudal part. The abdominal aorta is often smaller in diameter in comparison to the abnormal vessel, and often has insufficient branching.

Some authors consider sirenomelia to be a clinical manifestation of caudal regression syndrome because of abnormal development of fetal caudal mesodermal structures before the fourth week of gestation, that later extends to different craniocaudal levels [26]. It also leads to the absence of genitalia and renal agenesis if paramesonephric and mesonephric ducts are involved. If the mesonephric ducts had developed enough, and joined metanephric blastema, the kidneys may develop as well. Sometimes metanephric sites of mesoderm are damaged or underdeveloped, and in these cases hypoplastic or abnormal kidneys are formed [5].

Sonographic findings

The best method to diagnose the sirenomelia is the ultrasound examination – both 2D and 3D modes. The color and power Doppler is useful for the search of single umbilical artery, abnormal distal abdominal aorta and especially helps in cases of oligohydramnios. In the cases of oligohydramnios magnetic resonance imaging (МRI) [1,13,17,22,24,25,29,40,41,42,43,49,50,54,56,57,59] may also be very useful. Recently, thanks to the increase of combined first trimester examinations at 11+0 - 13+6 weeks of gestation, the diagnosis of sirenomelia can be easily done at the end of the first trimester comparing to the later pregnancy. Especially due to sufficient amount of the amniotic fluid, which is less dependent on the fetal urinary production in the first trimester [1,7,9,22,37,46,48,58]. Oligohydramnios developing later in the pregnancy due to a renal impairment makes diagnosis more difficult.

Cases of fixed lower extremities (with fusion of soft tissues of lower extremities), single lower extremity (bones fusion), and abnormal number of lower limbs are described [24, 49]. A single thick hip is described in cases of bone fusion [13]. Feet can be absent, entirely or partially fused into one foot of the unusual appearance.
The kidneys cannot be visualized usually due to the renal agenesia or dysgenesia [13,24,26,31,46,49,53,54,56.] The urinary bladder cannot be seen as well. Marked oligohydramnios is limiting factor of detection of other structural anomalies.

Joined femurs without their separate movements during ultrasound examination should give rise to a thought about possible lower limbs fusion. In the presence of oligohydramnios and bilateral renal agenesia, the observation of fused fetal lower extremities is a key to the antenatal diagnosis of sirenomelia.

Differential diagnosis

There is a controversy in the literature, whether sirenomelia as a separate entity or a severe form of the caudal regression syndrome [15,42,49,53,54,56]. However, the presence of two umbilical arteries, renal anomalies compatible with life, divided lower limbs, abdominal wall defects, anomalies of tracheoesophageal tree, neural tube, and heart, allows to differentiate the caudal regression syndrome and sirenomelia. Besides, the caudal regression syndrome shows continuous association with maternal diabetes mellitus  [20,25,33,43,56,63], and also with multiple pregnancy [2,3,8,16,34,39,47,56,63]. While performing differential diagnosis one should also exclude Potter syndrome, Meckel-Gruber syndrome, renal dysplasia, variants of obstructive uropathy, severe intrauterine growth retardation syndrome that is extremely difficult to do in second or third trimester because of severe oligohydramnios.

Associated anomalies

Since the reason for sirenomelia is an abnormal blood supply, they are lot of associated anomalies, making two identical cases looking different. Because of the vascular insufficiency and reduced blood supply to the caudal part of the fetus, anomalies of spinal cord, gastrointestinal and urogenital tracts usually take place: absence of kidneys, renal dysgenesia, imperforated anus, adrenal ectopia, single umbilical artery [2,10,30,36,43,46,52,54,56]. Some reports state that multicystic dysplastic kidneys accompany this pathology [24]. Lung hypoplasia and dolichocephalic configuration of the head with so-called Potter face is a consequence of oligohydramnios. Anomalies such as: abdominal cysts, malposition of visceral organs - situs inversus, umbilical hernia, oesophageal atresia [30,36,43,46,51,56]; absence of the upper extremities [8]; acardia and pentalogy of Cantrell [18,46,48,56,62]; anomalies of neural tube – spina bifida, hydrocephaly, anencephalia, cyclopia, cebocephalia, holoprosencephaly, meningomyelocele [11,27,28,36,40,44,46,47,56], and heart defects [54] were also described. Possible skeletal defects include: absence of sacrum, absence of the lower lumbar parts of spinal cord, segmental defects of distal parts of spinal cord.


Anomalies of kidneys, significant oligohydramnios and concomitant hypoplasia of the lungs make a very unfavorable prognosis for sirenomelia. Some milder forms of sirenomelia with survival have been described in the literature. Usually those had only soft-tissue fusion of the lower legs. The concomitant anomalies, such as anorectal defects, defects of urinary tracts, anomalies of pelvis and external genitals, have extremely poor prognosis and almost always lead to death or very significant disability [14,32,38,52,56].

Recurrence risk

Recurrence risk of sirenomelia is sporadic and fetal karyotype is usually unremarkable. This condition is considered to be 150 times more often observed at monozygotic twins, than in singleton pregnancies [3,46,50,56].


Termination of the pregnancy is an option due to a lethal prognosis. It should be offered to the parents when diagnosis of the sirenomelia is made.


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