Babies can be LBW for different reasons. First, the baby may be born too early. A typical pregnancy lasts about 40 weeks. If the baby is born before 37 weeks of pregnancy (gestation), it is called a preterm birth.
A baby may also be unable to grow enough before delivery. This is called inadequate fetal growth or intrauterine growth restriction. Such babies are described as small for gestational age (SGA), meaning smaller than usual considering the number of weeks of pregnancy.
A combination of the two reasons - being born too early and not growing enough before delivery - can also lead to LBW.
Low birth weight can be serious for new babies. Not only are they more likely to die in infancy, they may also experience health problems during infancy and childhood. In particular, babies born preterm or SGA who have a low birth weight are at an increased risk of health problems, including death. Brain development, vision and hearing difficulties can all be affected. Problems may arise with chronic respiratory conditions and infections. The earlier a baby is born, the greater the risk of both immediate and long-term health problems.
In Canada, the number of babies born weighing less than 2500 grams has remained pretty much the same over the last ten years. About 18,500 babies born in 2001 were LBW, around 5.5 per cent (one in 18) of all births.
While the number of LBW babies has remained fairly constant, the number of babies born early appears to be increasing. In Canada, the number of preterm babies climbed from 6.6 per cent in 1991 to 7.6 per cent in 2000.
Low birth weight can be caused by being born too soon (preterm birth), not growing enough before delivery (SGA), or a combination. So, the risk factors for LBW are very similar to those for a preterm and a small for gestational age baby. Some of the risks include:
Researchers have found that when a woman smokes during pregnancy, her baby suffers. Smoking increases the risk of growth restriction before birth, preterm birth and low birth weight. It also increases the risk of miscarriage, problems with the placenta, stillbirth (where the baby dies before birth), and sudden infant death syndrome. Nicotine narrows the blood vessels so that the blood carrying oxygen has a harder time getting through to the baby's developing organs. There is also less oxygen in the blood because of carbon monoxide in the smoke. It is hard for the baby to get enough oxygen to grow properly.
Still, about 20 per cent (one in five) of pregnant women in Canada report they smoke during pregnancy. That number is not much different than the overall smoking rate among all women.
Certain conditions, especially those affecting the reproductive system, are linked to preterm birth.
Infections - If the mother's genital system is infected, bacteria may weaken the fluid-filled amniotic sac that surrounds the baby. The sac may rupture early, leading to a preterm birth.
Abruptio placentae - Normally, the placenta is firmly attached to the uterus until the baby is born. With abruptio placentae, the placenta separates from the uterus too early. Since the placenta is a lifeline supplying nutrients and oxygen to the baby, this can lead to preterm birth and low birth weight. As well, the mother may experience major loss of blood.
Pre-eclampsia (also called toxemia or pregnancy-induced hypertension) - In this condition, the pregnant woman has high blood pressure, retains fluid, and has protein in her urine. It can be mild or severe. In more serious cases, restricted blood flow to the placenta can seriously harm the baby. Pre-eclampsia is more likely to occur in first pregnancies, usually after the twentieth week.
Incompetent cervix - Normally, the cervix (the part of the uterus at the top of the vagina) stays tightly closed during pregnancy. The baby's weight should not affect it. Sometimes, the cervix opens or dilates too early, allowing the amniotic sac and baby to exit the uterus prematurely.
Some evidence suggests that older mothers have a higher chance of preterm birth, multiple birth, and low birth weight. Over the years, more women have begun to start their families later in life. The birth rate for Canadian women 40 years or older has risen steadily over the last ten years, as has the age of women having their first baby. In 1986, the average Albertan woman's age at delivery was 26.7 years. In 2000, it was 28.3 years - almost two years older. Of all first time moms, nine per cent were 35 years or older in 2001, compared to only two per cent back in 1983.
On the flip side of the coin are the young moms - those who under the age of 20. Often, teens do not get prenatal care. They may wait longer into the pregnancy before seeing a doctor. Cigarette smoking, alcohol and other substance abuse is higher among some pregnant teens. As a result, teens are at higher risk of having a LBW baby, preterm birth, or SGA baby.
A pregnancy with more than one baby is often referred to as a multiple birth. Such babies are more likely to be preterm and growth restricted. The number of multiple births has been increasing in Canada for two reasons. More women aged 35 and older are having babies. More couples need fertility treatments and assisted conception to become pregnant. Putting off starting a family until later in life is a major cause of infertility, and is linked to fertility treatment. This, in turn, is linked to multiple births.
More recently, psychological factors like stress have been linked to preterm birth and LBW. The theory is that severe stress can lead to the release of hormones that trigger contractions of the uterus and early labour. Other risk factors, like smoking or specific medical conditions, can also exist in a pregnancy.
Health professionals have worked to prevent low birth weight in babies for many years. Now, we are aiming to prevent preterm birth and growth restriction in an effort to reduce the number of low birth weight babies. Many measures can help prevent preterm birth and growth restriction. Mothers, their families and health professionals all play a role in safeguarding the health of unborn babies. Mothers can make lifestyle changes such as eating well and quitting smoking. Families can support the pregnant mother and help reduce stress. Health professionals can provide good prenatal care and use obstetrical interventions where appropriate.
Sometimes, prevention programs work. At other times, they do not. However, discovering how to reduce the number of LBW babies continues to be an important question. Researchers, health care professionals and families can all work together to find answers. With prevention measures and ongoing research, hopefully fewer children will have to live with serious health problems.