Premature Baby-Definition and Background Information.

premature baby in NICU

The term prematurity is used to describe a baby born at more than three weeks early from its due date. In this article, I will talk about terms used to describe premature babies and give you basic statistical information about rates of premature births in the USA.

Definition of prematurity.

Prematurity is defined as a baby being born at less than 37 weeks of gestational age, or more than three weeks earlier than its due date, or at less than 259 days after a day of mother’s last menstrual period

If you are interested in finding out what is gestational age in weeks of your baby/fetus during your pregnancy, google for “gestational age pregnancy calculators,” and you will find plenty of resources. One of the websites with a calculator is available to you here.

If you have a premature baby born before 32 weeks receiving treatment in the NICU right now, I encourage you to explore also my book “Babies Born Early

What are the terms used to describe premature babies?

There are many qualifying terms used to describe the degree or severity of prematurity; however sometimes, definitions of those terms are not very precise. I will talk about the most frequently used ones here.

Mild prematurity – when a baby is born at GA more than 32 weeks but less than 37 weeks of gestational age and/or with BW above 1500 grams.

Moderate prematurity – when a baby is born between 28 weeks and 32 weeks of gestational age and with birth weight between 1000 to 1500 grams.

Extreme prematurity – baby is born at less than 28 weeks of gestational age and with a birth weight below 1000 grams. 

Additionally, with the increasing survival of very very small babies, some people started using the term “micro-preemie” – this term is generally used to describe babies born at less than 26 weeks of Gestational Age and with a birth weight of fewer than 800 grams.

In addition to terms describing the degree of prematurity, doctors use qualifiers, giving us information regarding the baby’s weight in relation to expected weights for the particular gestational age group. After birth, we plot the baby’s weight on a growth chart to find out whether the baby is AGA, SGA, or LGA. 

AGA means Appropriate Weight for Gestational Age and term is assigned to a baby when its weight is between 10th and 90th percentile on the growth chart. In other words, for all babies born at the same age as this AGA baby, 10% of babies will have weight below 10th percentile and will be called SGA or Small for Gestational Age, 80% of babies will have weights between 10th and 90th percentile and will be called AGA, and 10% of babies will have weights above 90th percentile and will be called LGA or Large for Gestational Age. 

Assigning SGA, AGA, or LGA classification to the baby’s weight helps anticipate specific problems that are more common to a particular group of babies. For example, for the same gestational age, SGA babies tend to have less severe breathing problems, but SGA babies will have more issues with regulation of body temperature, glucose levels, and proper brain development. 

The last term that I want to introduce to you is IUGR, which stands for Intrauterine Growth Restriction. IUGR is a diagnosis made by obstetrician based on several Ultrasound Examinations done during pregnancy. IUGR means that the baby’s weight does not follow initially assigned percentile rank. For example, at 12 weeks, fetus weight was at the 50th percentile for its gestational age, at 15 weeks weight was at 30th percentile, and at birth at 28 weeks weight was at the 20th percentile. Whenever we see such a pattern of falling off of initially assigned percentile rank, we talk about baby or fetus not fulfilling its initial growth potential. IUGR may be caused by maternal medical conditions, problems with the placenta or conditions originating in a baby.

What types of medical problems may premature babies have?

All premature babies may need evaluation and treatment after birth. However, extremely premature babies and micro-preemies are the most vulnerable. Frequently, they will have to stay in NICU for many weeks, months, or even a year. Those babies are born very small, fragile, and have numerous medical problems. 

I listed below just some of the problems for which we evaluate and treat premature babies in the NICU (My article introducing parents to the NICU environment). The type and number of issues that a baby gets will depend on the degree of prematurity and individual situation of each baby. Some babies may go through almost all of them, and some will have a relatively benign hospital course:

  • low body temperature
  • feeding problems
  • infections
  • respiratory problems (my article on RDS)
  • blood pressure problems
  • low sugar levels
  • jaundice – yellow skin (my article on jaundice)
  • chronic lung disease (=chronic oxygen dependence)
  • ROP or Retinopathy of Prematurity (Disease of the immature eyes that may lead to blindness)
  • NEC or Necrotic Enterocolitis (Inflammatory or infectious disease of immature intestines that may lead to perforation)
  • IVH or Intra-ventricular Hemorrhage (Bleeding in the brain)

In survivors, prematurity may lead to long-lasting effects such as developmental problems affecting their motor and cognitive performance. Those are critical issues and cause a lot of stress among the parents and families of premature babies. 

How common is premature birth? 

In 2017 in the US, we had 3.8 million live births. Out of those, 10% were born prematurely, meaning before 37 weeks of gestational age, and that comes to approximately 380 thousand premature babies per year. A staggering number! (Source article).

Unfortunately, there is a substantial racial disparity in rates of prematurity. Only 9% of white none-Hispanic babies are born prematurely, and almost 14% percent of black babies are premature at birth. The exact explanation for this disparity is still unknown.

Only 0.67% of all babies were born at less than 28 weeks of gestational age. However, still, it constitutes a large number of infants. Approximately 26 thousand of babies across all US were born at less than 28 weeks. 

What are the causes of premature birth?

Whenever I speak to parents of a premature baby for the first time, almost always, I am asked what caused it? Did we do anything wrong? I quickly reassure parents that it is nobody’s fault, and most importantly, it is not the mother’s fault that her pregnancy resulted in premature labor and birth. 

In 50% of cases, pregnant mothers go spontaneously into preterm labor. In 40%, they have preterm rupture of membranes that results in the birth of preemie babies. In 10% of cases, there are maternal medical indications to deliver the baby early. We also know from epidemiological studies that silent infections of the upper genital tract may be contributing factors to premature births. 

There are multiple risk factors cited in literature that are associated with the preterm delivery of the baby. They can be divided into four categories: demographic factors, behavioral factors, maternal medical conditions, and pregnancy complications. 

Among demographic factors, we know that women who are black, less than 17 y/o or more than 35 y/o, less educated, or from lower socioeconomic status have higher rates of premature births.

Among behavioral factors: poor nutritional status, low body mass index, smoking, substance abuse, lack of prenatal care, and stress contribute to higher rates of premature labor.

Maternal medical conditions such as uterine or cervical malformations, myomas, high blood pressure, diabetes are associated with higher rates of premature births.

Pregnancy characteristics such as twins or triplets, increased or decreased amount of amniotic fluid, vaginal bleeding, low body mass index, fetal anomalies, abdominal surgery, and infections are known to be associated with higher rates of premature births. 

I want to emphasize that the associations mentioned above do not mean a cause-effect relationship. If right now, you are pregnant and have one or even several risk factors that are associated with premature delivery, you should not panic. Make sure that you receive your prenatal care from the doctors who are familiar with taking care of high-risk pregnancy and can start appropriate monitoring and perhaps use some preventive measures. 

If you already had a premature baby in the past and you are planning to get pregnant in the future, you should find an obstetrician that will be monitoring you for early signs of premature labor. We can measure the cervix length using ultrasound techniques.  The finding of the shortened cervix may be associated with an increased risk of premature delivery. 

Research showed that if we prescribe progesterone to patients at an increased risk of premature labor, we can delay the occurrence of early labor and improve outcomes for these women and their babies.

In summary:

Premature babies are the newborns born at less than 37 weeks of gestational age. Prematurity is a common occurrence since one out of 10 babies is born prematurely. The severity of health problems and the number of complications will be highest for the smallest and tinniest babies who are called micro-preemies and extremely premature babies. Premature babies frequently have to stay in NICU for a very long time, and some of those babies will face chronic health problems. 


This article is only for general information purposes. It should not be viewed as any medical advice. There is a chance that information here may be inaccurate. It would be best if you always discussed all health-related matters with your doctor before making any decisions that may affect your health or health of your family members.



Dr.Wisniewski is a board-certified pediatrician and neonatologist with over 20 years of clinical experience in the USA. He authored the book: "Babies Born Early - A guide for Parents of Babies Born Before 32 Weeks" Dr.Wisniewski loves educating parents on various health conditions affecting their newborn babies and children.

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