In the United States, it has been estimated that by 2011, 36% of twin births and 78% of triplet and higher-order births resulted from conception by assisted reproductive technology. Twins
Twins are by far the most common form of multiple births in humans. The U.S. Centers for Disease Control and Prevention report more than 132,000 sets of twins out of 3.9 million births of all kinds each year, about 3.4%, or 1 in 30. Without fertility treatments, the probability is about 1 in 60; with fertility treatments, it can be as high as 20-25%.Dizygotic (fraternal) twins run in families; however, the male does not influence the chances of his partner having twins, even if there are twins in his family. The hyperovulation gene is the cause for having twins and is only a factor for the mother. Although the male does not influence his partners chances of having twins, he could influence his childrens chances for having twins. If a male carries the gene for hyperovulation and has a daughter, she may also have the gene for hyperovulation and then have twins herself. Monozygotic (identical) twins do not run in families. The twinning is random, due to the egg splitting, so all parents have an equal chance of conceiving identical twins. Triplets
Triplets can be either fraternal, identical, or a combination of both. The most common are strictly fraternal triplets, which come from a polyzygotic pregnancy of three eggs.
With such hyper-extensibility, it is also quite common (if not the norm) to find the muscles surrounding the joints to be a major source of compensation when such instability exists. Excessive lordotic curvature – lumbar hyperlordosis, is also called "hollow back", and "saddle back" (after a similar condition that affects some horses); swayback usually refers to a nearly opposite postural misalignment that can initially look quite similar. Common causes of lumbar hyperlordosis include tight low back muscles, excessive visceral fat, and pregnancy. Rickets, a vitamin D deficiency in children, can cause lumbar hyperlordosis. Lumbar hypolordosis
Being less common than lumbar hyperlordosis, hypolordosis (also known as flatback) occurs when theres less of a curve in the lower back or a flattening of the lower back. This occurs because the vertebrae are oriented toward the back of the spine, stretching the disc towards the back and compressing it in the front. This can cause a narrowing of the opening for the nerves, potentially pinching them.