What Are The Causes Of Premature Birth?

premature baby in NICU

Premature birth, which is defined as delivery of the baby before 37 weeks, occurs in approximately 10% pregnancies in the USA. The incidence will differ for different races and also vary depending on various risk factors. Interestingly, it is the lowest for white mothers and the highest for black parents. 

Some of the first questions parents ask of me after the delivery of a premature baby are: Is it my fault? Why did this happen to us? 

In this article, I will talk about the causes and factors associated with premature birth. I will also discuss if we can predict early labor and what your options are for prevention and treatment if you have multiple risk factors for having a premature baby. 

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” I this book I talk about many conditions that premature babies are treated for.

Causes of premature birth

Premature birth may occur spontaneously or may be medically indicated. Approximately 70% of premature births are spontaneous. They are caused by mother going into preterm labor (40%-50%), premature rupture of the amniotic sac (20%-30%), or more rarely cervical insufficiency. 

Cervical insufficiency can be described as the cervix getting weaker, thinner, and starting dilating during pregnancy without any contractions and signs of labor. When this condition affects woman significantly earlier than her due date, it will likely result in premature birth. 

About 30% of premature births are medically indicated. Therefore, either labor is induced with contractions stimulating medications or decision is made to do a cesarean section. 

In table 1, I listed some examples of conditions that may warrant induction of early delivery. The conditions are grouped into two categories: maternal and neonatal indications.

Table 1

Maternal ReasonsNeonatal reasons
High blood pressureMultiplets higher than twins
Placenta previa
Poor growth of the fetus
Abruptio placentaCongenital anomalies
Heart or kidney failureRH – incompatibility
CancersHydrops fetalis
Serious traumaSevere Arrhythmia

Risk factors associated with premature birth

In most situations, we do not know what triggered premature labor or premature rupture of membranes in any particular woman. However, there are multiple risk factors identified that are associated with premature birth. I want You to understand the crucial difference between risk factor and cause of something. The cause means that after an event, one should expect a certain specific outcome. For example, I cut skin with the knife, and I will bleed from the wound. 

Risk factor means only a loose association. For example, being a tall person is a risk factor for injuring your head in a submarine where there is not enough space to walk straight. But, not all tall people will hurt their heads in a submarine. In fact, some short people also will hurt their heads. 

Maternal demographic factors

Decades of epidemiological research provided us with the information that there are numerous demographic factors associated with premature birth. 

Some of those demographic factors are listed below:

  • mother’s age below 17 years or above 35 years
  • unmarried mother
  • education below high school level
  • low socioeconomic status (probably associated with an unhealthy lifestyle and lack of access to health care when needed)
  • short inter-pregnancy interval (less than six months) 
  • non-Hispanic black race
  • genetic factors

In one study, the interpregnancy interval of fewer than six months was associated with tripling the risk of delivering a baby at less than 34 weeks in a subsequent pregnancy. The risks are even higher if previous pregnancy ended in preterm birth as well (Source).

The organization that advocates for pregnant women, March of Dimes, suggests that if women want to minimize risks of premature delivery, the optimal interpregnancy interval should be around 18 months. 

It is not clear how the mother’s age influences the chances of delivering a premature baby. Some speculate that in young mothers, physiologic immaturity and socioeconomic factors play a role. On the other hand, older mothers have more preexisting conditions such as diabetes or obesity, which will have a significant impact on the date of delivery. 

In the USA non-Hispanic black parents have the highest rate of premature birth. Environmental and social factors can explain some of the differences, but it is hypothesized that genetic factors may play an important role too. 

There is no doubt that specific genes may influence the time of delivery. Some families and racial groups have higher rates of premature birth than others. Women who were born themselves prematurely or who have a first-degree relative who had a premature baby have higher rates of early delivery as well. 

Prior obstetrical and gynecological history

History of cervical surgery

Having a history of cervical surgery is associated with a higher risk for miscarriage or preterm delivery. Usually, cervical surgery procedures had to be done to treat cervical cancer and could not be avoided. 

Uterine malformations

Changes to the uterine environment and uterus shape such as the uterine septum, bicornuate uterus, or large fibroids, may also increase risks for miscarriage or preterm birth of the baby. Sometimes, those abnormalities can be corrected surgically before a woman gets pregnant. 

History of spontaneous preterm birth

Table 2 shows estimated risks of premature birth in subsequent pregnancy depending on whether previous pregnancy ended in premature delivery and at what gestational age premature labor took place (Modified from the Source).

Table 2

Prior birth at:Risk for birth < 28 wksRisk for birth < 32 wksRisk for birth < 37 wks
no prior early birth0.23%0.85%8.8%
at 23-27 weeks5%10%27%
at 28-34 weeks3%5%24%
at 35-36 weeks1%4%21%

History of having an abortion in the past

Women who had medically induced abortion tend to have the same level of risk for preterm labor as women who did not have an abortion. However, women who had a surgical abortion have slightly increased chances of delivering a premature baby in subsequent pregnancies. 

Nutritional status and physical activity of the mother

  • Body Mass Index < 19 or Weight < 50 kg
  • Poor nutritional status
  • A job requiring night shifts or standing for many hours

Women with normal nutritional status and mid-range body mass index have the best pregnancy results when term baby is considered as an ideal and most desired outcome of the pregnancy. 

A combined research study of different reports analyzing job conditions and the rates of premature birth concluded that the following types of job activities increase risks of having a premature baby (Source):

  • Standing and walking for more than 3 hours
  • Lifting more than 5 kg
  • Working night shifts
  • Working rotating shifts
  • Working more than 55 hours per week

Current maternal and pregnancy history

  • Multiplets higher than twins
  • Pregnancy as a result of IVF
  • Fetal chromosomal abnormalities
  • Fetal congenital abnormalities
  • Low amniotic fluid levels
  • High amniotic fluid levels
  • Maternal chronic medical conditions
  • Drugs, alcohol and tobacco use
  • Infections
  • Vaginal bleeding during the 1st or 2nd trimester
  • Abnormal placentation (previa or abruption)

It has been well established that women who suffer from 1st and 2nd trimester vaginal bleeding have higher chances of having premature rupture of membranes and placental abruption. Both can lead to early delivery of the baby. Some of these women may be candidates to be treated with progesterone in the affected or subsequent pregnancy.

Higher-order multiples are often associated with premature birth. It is most likely due to extreme distension of the uterus and overcrowding effects. 

Multiple scientists, including microbiologists and epidemiologists, reported on the association between infections and premature birth. That relationship is most likely mediated by inflammatory chemicals called prostaglandins. One study cited the presence of infection in up to 70% of placentas and membranes taken from mothers who had a premature baby. 

Treatment with antibiotics of any already identified infection in a woman is necessary. Pregnant women should be monitored and screened for the following infectious diseases during pregnancy: 

  • Bacterial vaginosis
  • Chlamydia and Gonorrhea
  • Urinary tract Infections
  • Syphilis and Hepatitis
  • Trichomonas
  • Herpes

Despite ample evidence of the association between infections and premature birth, studies that tried using antibiotics as a prophylactic measure aimed at preventing early labor failed to show positive results. 

Chronic maternal medical conditions that may increase the chances of delivering a premature baby are listed below:

  • High blood pressure (also called preeclampsia)
  • Gestational or preexisting diabetes
  • Hypothyroid disease
  • Chronic severe asthma
  • Chronic severe kidney disease

Aggressive treatment of those maternal conditions is of paramount importance in order to improve neonatal outcomes. 

Pregnancies that are a result of in vitro fertilization are at increased risk of resulting in the early delivery of the baby. That is true even for a singleton pregnancy. The increased risk may be due to maternal factors affecting the mother’s infertility or associations with side effects of assisted reproduction procedures. 

Can we predict the premature birth of the baby?

Risk scoring algorithms

Numerous researchers and centers tried developing risk scoring systems. They believed that identifying all risk factors for premature birth and assigning them individual weight may result in the final score that would identify women who will deliver a premature baby. 

Unfortunately, all scoring systems fail in predicting reliably who is at risk of having a premature baby and who is not. The reliability of those systems is poor and not that useful in clinical practice. In the clinic, we often see first-time mothers without any risk factors who end up having premature babies. Therefore, scoring systems would not have helped us in such situations. 

Fetal fibronectin

Fetal fibronectin is a chemical biomarker produced by fetal cells, and it may leak into the vagina when premature labor is likely to occur. An obstetrician can test for fetal fibronectin by swabbing a vaginal area and sending a sample to the laboratory. 

The laboratory can perform testing almost immediately. They can give us results back within an hour. A negative result usually means that there is less than a 5% chance that this pregnancy will result in the premature delivery of the baby in the next two weeks.

Measuring cervical length

Cervical length measurements conducted using a transvaginal ultrasound technique may assist us in the evaluation of risks for premature birth. 

It has been established that women with a longer cervix have a lower risk of premature birth. Some obstetricians recommend that all women should undergo ultrasound screening for short cervix at 18-24 weeks. They consider the cervix to be short when it is less than 25 mm in length. 

Depending on other obstetrical factors, women with a shortened cervix may be candidates for the treatment with cerclage or progesterone. 

Cerclage is a suture that can be surgically placed on the cervix to prevent the dilation of the cervix. 

Progesterone is a hormonal treatment that has been shown to prolong the duration of pregnancy in women with a shortened cervix. 

Can we prevent the premature birth of the baby?

There is no single and successful method of preventing premature birth. The reason for that is that we still do not know the exact causes of premature birth. We can modify some risk factors associated with the early delivery, thus improve neonatal outcomes. 

Easy targets that come to mind are treating all chronic maternal conditions thoroughly, ensuring proper access to health care, particularly for the women with risk factors, and eliminating harmful exposures to drugs, alcohol, or tobacco. 

Cervical cerclage

Cervical cerclage is a procedure during which a suture (stitch) is placed on and around the cervix to keep it closed and prevent premature delivery of the baby. It can be done through the vagina (more frequently) or during abdominal surgery. 

History of past miscarriages and premature births suggestive of the incompetent cervix or dilation and shortening of the cervix before 28 weeks without signs of labor are indications for cerclage placement. 

In most cases,  your obstetrician can do the procedure in his office or local hospital. 

Progesterone administration

Some studies showed that giving hormone progesterone to women with threatened premature birth may delay or prevent it. Usually, progesterone is given to women with a pregnancy between 20 – 28 weeks and evidence of short cervix. 

Progesterone can also be considered in women who have a history of pregnancy loss following vaginal bleeding in the first or second trimester. 

Steroid treatment

Steroids such as Bethametasone or Dexamethasone should be given to mothers who are likely to deliver a premature baby in the next seven days. Steroids do not modify the duration of the pregnancy, but they are given to improve the baby’s health outcomes after birth. 

It is recommended that we give steroids to any woman who is in labor or threatened labor before completed 34 weeks. 

There are several positive outcomes for the baby If at least 24-48 hours pass between administration of maternal steroids and the birth:

  • lower neonatal mortality
  • decreased incidence and severity of respiratory problems in a baby
  • reduction in the rate of intraventricular hemorrhage (bleeding in the brain – severe complication of prematurity)

Sometimes, steroids may increase maternal blood pressure and glucose levels. Concerns about the possibility of an increase in infections in mothers or their babies have been unfounded. Therefore it should not prevent us from giving steroids to mothers. 

Effects of repeated courses of steroids on premature babies’ long term outcomes are still under study. Therefore, we need to be cautious if we want to offer 2nd or 3rd round of steroids to mother who returns 2 or 3 weeks later again in premature labor.

You can learn about the problems premature babies have after birth from my article here.

What questions should you ask your obstetrician if you had a miscarriage or premature baby in the past

  • What caused it?
  • Do I have any chronic conditions that should be treated more aggressively to avoid another unfavorable outcome in the future?
  • What should I do in the future to minimize the chances that I will have a miscarriage or premature baby again?
  • Should I undergo any diagnostic workup?
  • If I get pregnant, should I get a referral to a high-risk pregnancy clinic?


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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