Premature birth

A premature birth is a birth that takes place more than three weeks before the baby is due. In other words, a premature birth is one that occurs before the start of the 37th week of pregnancy. Normally, a pregnancy usually lasts about 40 weeks.

Premature birth gives the baby less time to develop in the womb. Premature babies, especially those born earliest, often have complicated medical problems.

Depending on how early a baby is born, he or she may be:

  • Late preterm, born between 34 and 36 weeks of pregnancy
  • Moderately preterm, born between 32 and 34 weeks of pregnancy
  • Very preterm, born at less than 32 weeks of pregnancy
  • Extremely preterm, born at or before 25 weeks of pregnancy

Most premature births occur in the late preterm stage.

Symptoms Risk factors Complications Prevention

A premature birth means that your baby hasn't had the usual amount of time to develop in the womb before needing to adapt to life outside the womb.

The signs that a baby's gestation has been cut short include:

  • Small size, with a disproportionately large head
  • Sharper looking, less rounded features than a full-term baby's features, due to a lack of fat stores
  • Fine hair (lanugo) covering much of the body
  • Low body temperature, especially immediately after birth in the delivery room, due to a lack of stored body fat
  • Labored breathing or respiratory distress
  • Lack of reflexes for sucking and swallowing, leading to feeding difficulties

The following tables show the median birth weight, length and head circumference of premature babies at different gestational ages for each sex.

Weight, length and head circumference by gestational age for boys
Gestational ageWeight LengthHead circumference
40 weeks7 lbs., 15 oz.
(3.6 kg)
20 in. (51 cm)13.8 in. (35 cm)
35 weeks5 lbs., 8 oz.
(2.5 kg)
18.1 in. (46 cm)12.6 in. (32 cm)
32 weeks3 lbs., 15.5 oz.
(1.8 kg)
16.5 in. (42 cm)11.6 in. (29.5 cm)
28 weeks2 lbs., 6.8 oz.
(1.1 kg)
14.4 in. (36.5 cm)10.2 in. (26 cm)
24 weeks1 lb., 6.9 oz.
(0.65 kg)
12.2 in. (31 cm)8.7 in. (22 cm)
Weight, length and head circumference by gestational age for girls
Gestational ageWeightLengthHead circumference
40 weeks7 lbs., 7.9 oz.
(3.4 kg)
20 in. (51 cm)13.8 in. (35 cm)
35 weeks5 lbs., 4.7 oz.
(2.4 kg)
17.7 in. (45 cm)12.4 in. (31.5 cm)
32 weeks3 lbs., 12 oz.
(1.7 kg)
16.5 in. (42 cm)11.4 in. (29 cm)
28 weeks2 lbs., 3.3 oz.
(1.0 kg)
14.1 in. (36 cm)9.8 in. (25 cm)
24 weeks1 lb., 5.2 oz.
(0.60 kg)
12.6 in. (32 cm)8.3 in. (21 cm)

Premature babies can quickly develop serious complications, such as infection in the bloodstream (sepsis) and respiratory distress syndrome.

When to see a doctor

You'll be seeing members of the neonatal intensive care unit (NICU) often. Doctors often visit NICUs at a similar time each day. But, don't hesitate to ask questions even when there's not a regularly scheduled meeting, especially if your baby seems listless, has poor color, or refuses the bottle or breast after trouble-free feedings.

© 1998-2015 Mayo Foundation for Medical Education and Research (MFMER). All rights reserved. Terms of use