[1] It has also been classified as an expanded part of the VACTERL association and as a form of caudal regression syndrome.

It was previously thought to be a severe form of sacral agensis/caudal regression syndrome, but more recent research confirms that these two conditions are not related.

Other hypotheses involve an insult to the embryo between 28 and 32 days affecting the caudal mesoderm, a teratogen exposure affecting the neural tube during neurulation, and a defect in the twinning process that either stops the process of caudal differentiation or generates a second primitive streak.

[1][9][10] Though obvious at birth, sirenomelia can be diagnosed as early as 14 weeks gestation on prenatal ultrasound.

[1] Tiffany Yorks of Clearwater, Florida (May 7, 1988 – February 24, 2016)[13] underwent successful surgery in order to separate her legs before she was a year old.

[citation needed] Shiloh Jade Pepin (August 4, 1999 – October 23, 2009) was born in Kennebunkport, Maine, United States with her lower extremities fused, no bladder, no uterus, no rectum, only 6 inches of large intestine, no vagina, only one quarter of a kidney and one ovary.

She remained hopeful about her disease and joked often and lived her life happily, despite her challenges, as seen in her TLC documentary in "Extraordinary People: Mermaid Girl".

It seems that complications such as defects in the urogenital system as mentioned above can be possibly due to malformations in the intermediate mesoderm.

A four-hour operation to insert silicone bags between her legs to stretch the skin was successfully completed on February 8, 2005.

A successful operation to separate her legs to just above the knee took place May 31, 2005, in a "Solidarity Hospital" in the district of Surquillo in Lima.

The procedure, however, was so intensive that she became traumatized to the degree of losing her ability to form proper speech patterns, leaving her nearly mute.

[18] Her doctor Luis Rubio said he was pleased with the progress Milagros had made, but cautioned that she still needed 10 to 15 years of rehabilitation and more operations before she could lead a normal life, particularly reconstructive surgery to rebuild her rudimentary anus, urethra and genitalia.