If you’re worried about going into labor early or have delivered your new bundle of joy a bit earlier than expected, you’re not alone. About 10 percent of all births in the United States are premature (preterm) babies.

Sometimes babies are born prematurely for no reason at all. There is nothing you could have done differently, and your doctor could not have stopped it.

Premature babies can arrive 3 to almost 15 weeks earlier than the approximate due date at 40 weeks. How early a baby is born makes a difference in your baby’s health and in their birth weight.

Premature baby weight also affects how healthy your little one will be. The end of the third trimester is when babies gain weight quickly to get ready for birth. Some babies gain weight faster than others but an early delivery often means a small baby.

Remember, healthy babies come in all different sizes. However, babies that are born too tiny may face more challenges and need to do some growing before they can go home.

Fortunately, neonatal (newborn) care and incubators can help most premature babies catch up in growth and development. Babies born as early as 25 weeks have a survival rate of up to 81 percent, while a baby born at about 34 weeks can have a survival rate of 100 percent.

In the third trimester your baby is finishing off developing and the focus turns to gaining weight. At about 31 weeks of pregnancy babies begin to gain weight very quickly. A baby can more than double their weight in only about 10 weeks.

Your little one weighs only about 3 pounds at week 30 of pregnancy. Most full-term babies born at about 40 weeks of pregnancy weigh an average of 7 1/2 pounds. This is why the earlier a premature baby is born, the smaller and lighter they might be. However, even full-term babies can also sometimes have low birth weight.

In the United States, about 8 percent of babies have low birth weight. This means that they are born weighing at or less than 5 pounds, 8 ounces. Only about 1.4 percent of all babies in the United States are born at a very low birth weight of less than 3 pounds.

Many factors can influence your premature baby’s weight including pregnancy complications, infections, and maternal behaviors. But one of the primary factors in an early arrival and lower birth weight is carrying more than one baby.

Expecting multiples

If you’re pregnant with twins, triplets or other multiples, your babies have a higher chance of being born early and on the smaller side.

In the United States, only about 2 percent of singleton babies are born early preterm, or at less than 34 weeks. That percentage jumps to almost 20 percent for twin babies and up to 63 percent for triplets.

Having quintuplets (5 babies) or more in one pregnancy? There’s almost 100 percent chance that your babies will be born early preterm.

The more babies you are carrying in one pregnancy, the shorter your pregnancy will likely be. This is simply because the babies outgrow your uterus (womb) faster. More babies sometimes also means more complications during pregnancy.

Twins, triplets and other multiples of babies are also more likely to have a lower birth weight than singleton babies. Twin babies are up to 55 percent more likely to weigh less than 5 1/2 pounds, compared to only 6.6 percent of single babies. Triplet babies have up to a 95 percent chance of being having low birth weights.

Other causes of low birth weight

Some babies may be born only slightly early or even full term and still have low birth weight. There are not usually any symptoms that your baby will be born low birth weight. This is why going to all prenatal checkups is so important.

Issues during a pregnancy can sometimes slow down a baby’s growth. This might happen due to:

  • problems with the placenta inside the womb
  • the mother’s health
  • a health condition in the baby

A pregnant woman’s bacterial or viral infection can also cause low birth weight. These include:

  • cytomegalovirus
  • toxoplasmosis
  • rubella
  • syphilis

Other causes of low birth weight include:

  • mother’s age (under 17 years or over 35)
  • smoking during pregnancy
  • drinking alcohol while pregnant
  • using drugs before or during pregnancy
  • heart disease
  • high blood pressure
  • some autoimmune diseases
  • poor nutrition before and during pregnancy
  • poor healthcare during pregnancy
  • exposure to lead
  • problems in the shape or size of the uterus (womb)

There are also genetic factors. If one or both parents are small or your previous babies were born at a lower birth weight it’s more likely your baby will have a lower birth weight.

The lower the birth weight, the higher the chance that a baby may have complications including long-term health problems. Premature babies may need medical care right away to treat:

  • low blood sugar levels
  • low oxygen levels
  • breathing problems
  • low body temperature
  • infections
  • difficulty feeding
  • difficulty gaining weight
  • bleeding problems
  • digestive problems

Very low birth weight (about 3 pounds) babies have the highest risks of long-term health issues, such as problems with:

  • vision
  • hearing
  • breathing
  • learning
  • digestion

These very low birth weight babies may also have

Low birth weight babies are usually treated in the neonatal (newborn) intensive care unit (NICU) immediately after birth. Treatment includes:

  • oxygen
  • help with breathing
  • temperature controlled incubator
  • special feeds sometimes with a tube
  • vitamin A and other nutritional supplements

Many babies who are premature also have trouble with feedings and coordinating their sucking and swallowing. This can mean a longer process to achieve weight gains.

Your baby will be weighed and have their height measured. Your doctor will also check the circumference (size) of their head. A baby’s head size is an important sign of good growth and development after birth.

Most premature babies with low birth weight catch up in weight and growth by the time they’re about between 18 and 24 months old. Your baby’s pediatrician will chart your baby’s weight gain against other babies of the same age and sex, to show how well they are doing.

Premature babies usually have low birth weight. Premature or preterm births are most common with twin and multiples pregnancies. The more premature your baby is born, the more likely they’ll have a lower birth weight.

Lower birth weights increase the chances of complications and some long-term health problems. However, it’s important to remember that, while you can’t control every factor, making healthy choices during pregnancy and keeping up with prenatal care helps to give your baby their best start.