Signs and symptoms of Prader-Willi syndrome can vary among individuals. Symptoms may slowly change over time from childhood to adulthood. Infants Signs and symptoms that may be present from birth include: 
Poor muscle tone. A primary sign during infancy is poor muscle tone (hypotonia). Babies may rest with their elbows and knees loosely extended instead of fixed, and they may feel floppy or like rag dolls when they're held.
Distinct facial features. Children may be born with almond-shaped eyes, a narrowing of the head at the temples, a turned-down mouth and a thin upper lip.
Poor sucking reflex. Infants may have a poor sucking reflex due to decreased muscle tone. Poor sucking makes feeding difficult and can result in failure to thrive.
Generally poor responsiveness. A baby may seem unusually tired, respond poorly to stimulation, have a hard time waking up or have a weak cry.
Underdeveloped genitals. Males may have a small penis and scrotum. The testicles may be small or not descended from the abdomen into the scrotum (cryptorchidism). In females, the clitoris and labia may be small.
 Early childhood to adulthood Other features of Prader-Willi syndrome appear during early childhood and remain throughout life, requiring careful management. These features may include: 
Food craving and weight gain. A classic sign of Prader-Willi syndrome is a constant craving for food, resulting in rapid weight gain, starting around age 2 years. Constant hunger leads to eating often and consuming large portions. Unusual food-seeking behaviors, such as hoarding food, or eating frozen food or even garbage, may develop.
Underdeveloped sex organs. A condition called hypogonadism occurs when sex organs (testes in men and ovaries in women) produce little or no sex hormones. This results in underdeveloped sex organs, incomplete or delayed puberty, and in nearly all cases, infertility. Without treatment, women may not start menstruating until their 30s or may never menstruate, and men may not have much facial hair and their voices may never fully deepen.
Poor growth and physical development. Underproduction of growth hormone can result in short adult height, low muscle mass and high body fat. Other endocrine problems may include underproduction of thyroid hormone (hypothyroidism) or central adrenal insufficiency, which prevents the body from responding appropriately during stress or infections.
Cognitive impairment. Mild to moderate intellectual disability, such as issues with thinking, reasoning and problem-solving, is a common feature of the disorder. Even those without significant intellectual disability have some learning disabilities.
Delayed motor development. Toddlers with Prader-Willi syndrome often reach milestones in physical movement — for example, sitting up or walking — later than other children do.
Speech problems. Speech is often delayed. Poor articulation of words may be an ongoing problem into adulthood.
Behavioral problems. Children and adults may at times be stubborn, angry, controlling or manipulative. They may throw temper tantrums, especially when denied food, and may not tolerate changes in routine. They may also develop obsessive-compulsive or repetitive behaviors, or both. Other mental health disorders, such as anxiety and skin picking, may develop.
Sleep disorders. Children and adults with Prader-Willi syndrome may have sleep disorders, including disruptions of the normal sleep cycle and a condition in which breathing pauses during sleep (sleep apnea). These disorders can result in excessive daytime sleepiness and worsen behavior problems.
Other signs and symptoms. These may include small hands and feet, curvature of the spine (scoliosis), hip problems, reduced saliva flow, nearsightedness and other vision problems, problems regulating body temperature, a high pain tolerance, or a lack of pigment (hypopigmentation) causing hair, eyes and skin to be pale.
