Chronic constipation is especially common,: 377  and is thought to be aggravated by an asymmetric pelvis (acetabular protrusion). : 377  Especially in childhood, OI-associated constipation may cause a feeling of fullness and associated food refusal, leading to malnutrition. : 377

Classification
There are two typing systems for OI in modern use. The first, created by David Sillence in 1979, classifies patients into four types, or syndromes, according to their clinical presentation, without taking into account the genetic cause of their disease. : 114–115  The second system expands on the Sillence model, but assigns new numbered types genetically as they are found. Therefore, people with OI can be described as having both a clinical type and a genetic type, which may or may not be equivalent.Type I is the most common, and 90% of cases result from mutations to either COL1A1 or COL1A2. Symptoms vary a lot between types, as well as vary from person to person, even in the same family.As of 2021, 21 types of OI have been defined:

Sillence types
Sillences four types have both a clinical and a genetic meaning; the descriptions below are clinical and can be applied to several genetic types of OI. When used to refer to a genetic as well as a clinical type, it indicates that the clinical symptoms are indeed caused by mutations in the COL1A1 or COL1A2 genes which are inherited in an autosomal dominant fashion. Type I
Collagen is of normal quality but is produced in insufficient quantities.
: 100, 1513 The disability rights critique of prenatal screening for OI, held by some bioethicists and some affected individuals, negatively compares it to eugenics, with even those not opposed to abortion opposing selective abortions on the ethical ground that their existence betrays the belief that the lives of those with OI are "less worth living [and] less valuable". : 388

Prognosis
The prognosis of osteogenesis imperfecta depends entirely on its type (see § Classification). Life expectancy
In the mild form of the disorder, type I, the life expectancy of patients is near that of the general population. : 461  In type II, however, patients only very rarely live past the age of two, and typically die in their first weeks of life. : 1511  Assessment of the life expectancy of patients with types III and IV is more complicated, as lifestyle choices can cause fatally traumatic injuries that would not have otherwise occurred, or not been fatal in the general population. Life expectancy in type IV OI is thought to be close to normal, but in type III it is lower than in the general population.A 2016 study of data in Denmarks National Patient Register found that across all types of OI, all-cause mortality was three times higher, leading to a loss of around seven years in females and nine years in males.