Sirenomelia is a rare malformation. It is caused by a primary defect of the caudal axial skeleton and damage to the primary streak, which appears due to a vascular steal phenomenon. It was first described in 1542 and compared to mermaid because of its characteristic appearance: a complete fusion of soft tissue in which both lower limbs are contained in a single skin sheath. This fusion may be partial or total. Sirenomelia appears with an incidence of 1 per 600,000 births. Diagnosis of sirenomelia in antenatal period by ultrasound in the first trimester is primordial so an interruption of the pregnancy can be proposed. A risk for sirenomelia can be also found in patients with poorly controlled diabetes mellitus [1] [2] . It’s a lethal condition due mostly to associated visceral abnormalities [3] . In this article, we report a case of Sirenomelia diagnosed after emergency delivery.

A 20 years old primigravida mother was referred at 35 weeks of gestation to the Department of Gynecology and Obstetrics of the University Hospital Souissi (Rabat, Morocco), with a suspect Intrauterine growth restriction (IUGR) associated to anamnios. She hadn’t any pregnancy follow-up and no antenatal ultrasound. Her past medical records showed type 1 diabetes for 12 years with insulin protocol. Her glycemic control during the pregnancy and before was not satisfying and her glycated hemoglobin at 6 weeks of gestation was at 8%.

She was admitted into the hospital for early labor with 3 cm dilatation and closer contractions. An ultrasound confirmed the anamnios with an estimated weight at 1800 g and a transversal presentation but no further abnormalities. She was then admitted to the OR for an emergency c-section and the delivery of a newborn with Apgar score 02 then died at 15 minutes of extra-embryonic life.

The examination at birth ( Figure 1 ) showed a single lower limb and a single rudimentary foot. In addition, there was an esophageal atresia, an anal atresia and an unidentifiable external genitalia. Also, the examination ( Figure 2 ) of the umbilical cord showed 1 umbilical artery and 1 umbilical vein. A radiography ( Figure 3 ) was performed in post-mortem showed that there was a femur and 2 long bones. Unfortunately, the parents refused an autopsy for further examination.

The prevalence of sirenomelia is estimated to be 1 per 60,000 births [4] [5] with a sex ratio of 2 boys affected for every girl [4] - [6] . This condition is typically accompanied by other abnormalities, including a single umbilical artery and malformations such as anal atresia, renal agenesis, urogenital and esophageal atresia [3] - [10] . During the second trimester, an oligohydramnios may occur due to these malformations, leading to pulmonary hypoplasia.

Sirenomelia is classified into seven types according to Stocker and Heifetz [11] .

The etiopathogenesis of this condition remains unclear. There is no evidence of chromosomal aberrations in humans or familial recurrences [8] . However, two main theories have been proposed in the literature. The first, proposed by Stevenson et al. in 1986, is the vascular steal theory [7] [8] , which suggests that the presence of a single umbilical artery redirects blood flow away from the caudal mesoderm of the embryo during early embryogenesis (between 13th and 22th days of gestation), resulting in agenesis of midline structures and fusion of the lower extremities. The second theory involves defective blastogenesis during the third week of gestation, leading to caudal regression syndrome and interfering with the development of caudal mesoderm [3] [7] - [9] .

In addition, certain risk factors may contribute to the development of Sirenomelia, including maternal diabetes and exposure to teratogenic agents during pregnancy, such as Retinoic acid, vitamin A, certain drugs, and cocaine [3] .

Diagnosis of sirenomelia during the antenatal period can be challenging in the second or third trimester due to oligohydramnios. However, diagnosis is more feasible during the first trimester using transvaginal ultrasound, which can detect limb and visceral anomalies [12] . Furthermore, color and power Doppler imaging are critical for evaluating the vascular abnormalities [13] .

Post-mortem autopsy remains a valuable method for studying fetal malformations. Some researchers have suggested imaging techniques, such as computed tomography (CT) and magnetic resonance imaging (MRI), may serve as alternatives to conventional autopsy, offering improved evaluation of internal organs [14] .

Misdiagnosis of sirenomelia can lead to the continuation of pregnancy, giving families false hope for fetal survival and potentially depriving them of the option to terminate the pregnancy during its early stages.

Healthcare professionals must prioritize proper care for pregnant women, particularly in populations with limited access to regular prenatal monitoring. Emphasis should also be placed on the critical importance of first-trimester morphological ultrasound in identifying anomalies.

Sirenomelia is a rare condition and the etiopathogenesis is still unclear. The diagnosis is made by an ultrasound in the first trimester to give the patient the choice of terminating the pregnancy at an early gestational age.

Written informed consent was obtained from the patient(s) for their anonymized information to be published in this article.

No funding or grant support.