A few quotes I've found following a quick google
Roy M. Nuzzo, MD, FAAOS,FAAOP, POSNA, New Jersey, USA
Cultures that carry babies with the child’s legs spread apart, such as riding on the mother’s hip, have, by far, the lowest incidence of hips that go on to be bad. African mothers have the best track record of all. Their protective way of carrying babies is unbeaten – held in an open-legged position by a sling-bandana. Cultures that wrap legs together in one way or another (swaddling, etc.) just plain destroy hips. Swaddling of children leads to waddling adults.
Prof. Dr. med. Büschelberger (Dresden)
“The head of the thighbone in a newborn is ideally orientated towards the hip socket – and thus fills it out evenly-when the legs are pulled up to approx. 100 degrees and spread approx. 40 degrees at the same time.
When infants are picked up they take up this posture spontaneously and therefore it has to co-operate with their anatomic conditions.
As well, the rounded back (kyphosis) doesn’t stretch out immediately after birth. On the contrary, the well-known double-s-form of the spine is developing fully at the beginning of self-supported walking.
These anatomic conditions, together with the palmar grasp reflex of the first days in life, suggest that infants are especially adapted to being carried on our body.
Thus, it is possible to make a statement about the ideal carrying position: with wide-spread,flexed legs, slightly rounded back and orientated towards the carrying adult."
And a crap-load of references from an article about babywearing
SCIENTIFIC RESEARCH
Perinatol, J. (1991). Current knowledge about skin-to-skin (kangaroo) care for pre-term infants. Journal of Perinatology 11(3), 216-226.
Anisfeld, E., Casper, V., Nozyce, M., & Cunningham, N. (1990). Does infant carrying promote attachment? An experimental study of the effects of increased physical contact on the development of attachment.
Hunziker, U. A., & Barr, R. G. (1986). Increased carrying reduces infant crying: A randomized controlled trial. Pediatrics, 77(5): 641-648.
Barr, R., et al. (1991). Crying in !Kung San infants. Developmental Medicine and Child Neurology, 33, 601-610.
Bergman, N. J., Linley, L. L., & Fawcus, S. R. (2004). Randomized controlled trial of skin-to-skin contact from birth versus conventional incubator for physiological stabilization in 1200- to 2199-gram newborns. Acta Paediatrica, 93, 779-785.
Carter, S. C. (1998). Neuroendocrine perspectives on social attachment and love. Psychoneuroendocrinology, 23, 779-818.
Charpak, N., Ruiz-Pelaez, J. G., Figueroa, Z., & Charpak, Y. (1997). Kangaroo mother versus traditional care for newborn infants <2000 grams: A randomized, controlled trial. Pediatrics, 100(4), 682-688.
Chomosky, N.(1998). Language and problems of knowledge. Cambridge: MIT Press.
K Christensson, T Cabrera, E Christensson, K Uvnäs–Moberg, J Winberg, Karolinska (1995) Separation distress call in the human neonate in the absence of maternal body contact
Christensson, K., Siles, C., Moreno, L., Belaustequi, A., de la Fuente, P., Lagercrantz, H., Puyol, P., & Winberg, J. (1992). Temperature, metabolic adaptation and crying in healthy full-term newborns cared for skin-to-skin or in a cot. Acta Paediatrica, 81, 488-493.
Douglas, P. S. (2005). Excessive crying and gastro-oesophageal reflux disease in infants: Misalignment of biology and culture. Journal Title, volume(issue), page numbers.
Durand, R., Hodges, S., LaRock, S., Lund, L., Schmid, S. Swick, D., et al. (1997). The effect of skin-to-skin breast-feeding in the immediate recovery period on newborn thermoregulation and blood glucose values. Neonatal Intensive Care, 10, 23-29.
Ruth Feldman 1 , Aron Weller 1 , James F. Leckman 2 , Jacob Kuint 3 & Arthur I. Eidelman (1994) The Nature of the Mother’s Tie to Her Infant: Maternal Bonding under Conditions of Proximity, Separation,and Potential Loss
Gerber, M. (1958). The psycho-motor development of african children in the first year and the influence of maternal behavior. Journal of Social Psychology, 47, 185-195.
Gray, L., Watt, L., & Blass, E. M. (2000). Skin-to-skin contact is analgesic in healthy newborns. Pediatrics, 105, 14.
Hensinger, R. N. (1989). Spondylolysis and Spondylolisthesis in children and adolescents. Journal of Bone and Joint Surgery, 71A, 1098-1107.
Insel, T. R. (1997). A neurobiological basis of social attachment. American Journal of Psychiatry, 154, 726-735.
Lamb, M. E. (1982). Individual differences in infant sociability: Their origins and implications for cognitive development. In H.W. Reese & L. P. Lipsitt (Eds.), Advances in child development and behavior (Vol. 16, pp. 213-239). New York: Academic.
Landau, R. (1982) Infant crying and fussing: Findings from a cross-cultural study. Journal of Cross-Cultural Psychology, 13, 427-444.
Littlefield, T. R. (2003). Car seats, infant carriers, and swings: Their role in deformational plagiocephaly. Journal of Prosthetics & Orthotics, 15(3), 102-106.
Ludington-Hoe, S. M., Lewis, T., Morgan, K., Cong, X., Anderson, L., & Reese, S. (2006). Breast-infant temperature synchrony with twins during shared Kangaroo Care. Journal of Obstetric, Gynecologic, and Neonatal Nursing, 35, 1-9.
Messmer, P. R., Rodriguez, S., Adams, J., Gentry, J. W., Washburn, K., Zabaleta, I., & Abreu, S. (1997). Effect of Kangaroo care on sleep time for neonates. Pediatric Nursing, 23(4), 408-414.
Meyer, K., & Anderson, G. C. (1999). Using kangaroo care in a clinical setting with full-term infants having breastfeeding difficulties. American Journal of Maternal Child Nursing, 24, 190-192.
Moore ER, Anderson GC, Bergman N. (2007) Early skin-to-skin contact for mothers and their healthy newborn infants.
Pelaez-Nogueras, M., Field, T. M., Hossain, Z., & Pickens, J. (1996). Depressed mothers’touching increases infants’ positive affect and attention in still-face interactions. Child Development, 67, 1780-1792.
Schmid, S., Swick, D., Yates, T., & Perez, A. (1997). The effect of skin-to-skin breast-feeding in the immediate recovery period on newborn thermoregulation and blood glucose values. Neonatal Intensive Care, 10, 23-29.
Schön, R., & Silvén, M. (2007). Natural parentingundefinedBack to basics in infant care. Evolutionary Psychology, 5(1), 102-183. https://www.epjournal.net/filestore/ep05102183.pdf
Tessier R., Cristo, M., Velez, S., Giron, M., Ruiz-Palaez, J. G., Charpak, Y., et al. (1998). Kangaroo mother care and the bonding hypothesis. Pediatrics, 102:17.
Wahlberg, V., Affonso, D., & Persson, B. (1992). A retrospective comparative study using the kangaroo method as a complement to standard incubator care. European Journal of Public Health, 2(1), 34-37.
Wall-Scheffler, C. M., Geiger K., & Steudel-Numbers, K. L. (2007). Infant carrying: The role of increased locomotory costs in early tool development.
Skin contact (kangaroo care) shortly after birth on the neurobehavioral responses of the term newborn: a randomized, controlled trial. Pediatrics 113(4):858-65.
I just think that I wouldn't want to to hung by my crotch dangling off someone's chest personally, so why would my baby want to be in that position? You probably won't carry your LO for long enough to do any harm (if indeed you believe FFO causes harm anyway) but to have 15lbs and above hanging away from your body and screwing with your centre of gravity may well do you some harm!