They weigh an average of a half-pound less. They are also more likely to be born early and need special care after birth. Smoking nearly doubles a woman’s risk of having a baby who weighs less than 5.5 pounds. Low-birth-weight babies who are born to women who smoke are at higher risk for illness and death.

The average length of pregnancy is 40 weeks. A preterm baby is born before 37 weeks. Cigarette smoke has about 2,500 chemicals in it. Nicotine and carbon monoxide are 2 of the chemicals that may harm the developing baby. These chemicals can keep food and oxygen from reaching the developing baby.

Children of mothers who smoke may lag behind in school. They may also be smaller than children whose mothers don’t smoke. That’s because babies born to mothers who smoke during pregnancy and who are exposed to secondhand smoke after birth can have weaker lungs. Secondhand smoke is smoke from a burning cigarette. It can also be smoke that a smoker breathes out. A baby’s lungs and airways are small. Breathing smoke-filled air makes it difficult for the baby to breathe. It can cause lung problems such as pneumonia and bronchitis.

SIDS is the leading cause of death in babies younger than 12 months of age. SIDS is the unexplained death of an infant. Being around smoke is a risk factor for SIDS.

Studies suggest that regular exposure to secondhand smoke may slow down the growth of the fetus. This makes it more likely for the woman to have a low-birth-weight baby, the CDC says.

Babies born to women who smoke during pregnancy are more likely to have these birth defects:

• Clubfoot
• Opening in the lip (cleft lip)
• Missing fingers or toes
• Opening in the roof of the mouth (cleft palate)

Smoking increases the risk for an ectopic pregnancy. This is when the embryo becomes implanted in a fallopian tube or another place outside the uterus.

Nicotine can be passed on to a baby through breastmilk.