Yep, giving birth to big babies can be hereditary. In general, babies tend to be in the same weight range as their parents. In other words, if you were nine pounds, eight ounces at birth, it’s incredibly unlikely that you’ll give birth to a five-and-a-half-pound peanut.

Causes. A person may be underweight due to genetics, improper metabolism of nutrients, lack of food (frequently due to poverty), drugs that affect appetite, illness (physical or mental) or the eating disorder anorexia nervosa.

A child inherits half their genes from their mother and half from their father, and the child’s own resulting genetic make-up plays a role in birth weight.

Summary: People who start adulthood with a body mass index (BMI) in the normal range and move later in life to being overweight – but never obese – tend to live the longest, a new study suggests. Adults in this category lived longer than even those whose BMI stayed in the normal range throughout their life.

Many underweight people are physically healthy. Low body weight is due to a variety of causes, including: Genetics. If you’ve been thin since high school and it runs in your family, it’s likely that you were born with a higher-than-usual metabolism.

Some studies have suggested it’s better to be thin rather than active. In one such study, women who were thin yet inactive had a lower chance for early death than those who had obesity and were active. However, in patients with heart disease, being physically active, was more beneficial than having a low BMI.

According to the current Dietary Guidelines for Americans a body mass index below 18.5 falls outside the healthy range. But some people manage to live long, healthy lives with a low body mass index. Here’s a good rule of thumb: If you’ve always had a low body mass index and your weight doesn’t change, don’t worry.