Preterm Birth. Reasons and Risks

November is prematurity awareness month, and I felt the need to shed the light on its importance from an obstetric instead of a neonatal point of view, but before talking about preterm births, let us define a “full term pregnancy”.

A full term pregnancy means a pregnancy ending at or after 40 weeks of gestation, and this number is calculated from a preset due date. But how is this date even set?
In the setting of a spontaneous pregnancy, a due date is either calculated from the date of last menstrual period (using Naegele’s formula) or based on the baby’s measurements on ultrasound during early pregnancy. The latter method is becoming the norm now (especially in eminent centers around the world), because the date of the last menstrual period is not helpful in dating a pregnancy in women with late ovulations or irregular cycles.
In the setting of a pregnancy following in vitro fertilization, the dating depends on the embryo transfer date and the age of the embryo at the time of the transfer. Regardless of the method used to date the pregnancy, once a due date is set it should never change, because timing the pregnancy is essential for obstetricians to monitor the fetal development, diagnose intrauterine growth restriction and decide on the timing of delivery.

Regardless of these details, a full term pregnancy spans over a period of 40 weeks, and it is divided into 3 trimesters; the first starts at conception till 13+6 weeks, the second from 14 till 27+6 weeks and the third from 28 weeks till delivery. Any birth before 37 weeks is labeled as preterm birth.

The embryo, and later on the fetus, needs the pregnancy period to develop, grow and mature in order to survive outside the womb after birth. Hence, any preterm delivery signifies the birth of an immature baby prone to postnatal complications.

Around 11% of births are preterm, corresponding to more than 15 million annual births worldwide. Most occur between 32 and 36+6 weeks of gestation, and around 15% occur before 32 weeks, or even before 28 weeks.
Depending on the gestational age at the time of delivery, a preterm birth is divided into 4 categories:

• Late preterm (34-36+6 weeks)
• Moderate preterm (32-33+6 weeks)
• Very preterm (VPT) (28-31+6 weeks)
• Extremely preterm (EPT) (before 28 weeks)

Keep in mind that the gestational age of a preterm delivery is directly proportional to the neonatal complications, and inversely proportional to the baby’s survival rate. I will not go through the details of postnatal complications as this would be a pediatric/neonatal subject, yet I will be focusing on exploring the risk factors of a preterm birth from an obstetric point of view.

So, why does this happen in some women?

80% of preterm births occur spontaneously after preterm labor or rupture of membranes, and 20% are medically indicated for maternal and/or fetal conditions.

The following are risk factors that were shown to increase preterm birth rates.

History of preterm birth

One of the most important risk factors for a preterm birth is a history of a prior preterm birth, actually after one preterm delivery the chance of recurrence is around 15%, after two preterm births this risk increases to 40%, and after three preterm births the chance of recurrence is approximately 75%. Hence, women with a prior preterm delivery should seek preconception counseling and proper follow-up with a specialized high risk obstetrician whenever attempting a subsequent pregnancy, because some early interventions were shown to reduce recurrence risk.

Maternal age

Teenage pregnancies are associated with a higher risk of preterm birth. Actually it has been reported that girls under the age of 16 have a 4% higher risk of preterm birth compared to women aged 21-35. This trend is also observed in women older than 35. However, keep in mind that being over the age of 35 is not a guarantee for a bad pregnancy outcome, as most pregnancies after 35 go by smoothly without complications.

Twins and triplets

Multiple pregnancies are known to be associated with higher spontaneous and medically indicated preterm delivery rates. More than 50% of twins and more than 90-95% of triplets are born prematurely. Of course in many instances the delivery is medically indicated for fetal or maternal well being (complicated twin pregnancies or twins sharing the same sac should be delivered prematurely to prevent prenatal fetal demise)

Preeclampsia

Briefly, preeclampsia is a serious obstetric condition characterized by elevated blood pressure during pregnancy with proteinuria (proteins in urine) or organ damage. This condition has different degrees of severity and depending on the later, the attainable gestational age for delivery is decided. Uncontrolled preeclampsia is associated with maternal and fetal complications, including death, and the only treatment of this condition is ending the pregnancy by giving birth. 

Infections

Infections have been associated with a higher preterm birth rate as well. Vaginal, urinary and uterine infections can cause preterm uterine contractions, cervical dilation and eventual delivery. This is understandable, yet it is not the end of the story, as women with infections elsewhere in their bodies can also end up with a preterm birth, and this is secondary to the inflammatory molecules that can trigger uterine contractions. A simple tooth infection or gingivitis during pregnancy can cause preterm labor and delivery, thus I encourage all women to have a dental check twice yearly, and especially before attempting getting pregnant (check out my article “Plan for your Pregnancy” on https://onlinewomenhealth1.blogspot.com/2020/06/plan-your-pregnancy.html)

Ethnicity

Non-Hispanic black women have the highest rate of preterm birth (14-15%) and whites have the lowest rates (9%). This difference is related to many genetic, environmental and socio-demographic discrepancies between different ethnic groups.

Maternal diseases

Many maternal conditions, especially if poorly controlled, have been associated with a higher preterm delivery rate, such as anemia, pre-existing diabetes, hypertension, thyroid disease or other. That’s why, I always encourage all women trying to get pregnant to plan their pregnancies and visit their physicians for preconception counseling before a positive pregnancy test. It is of utmost value to control your pre-existing chronic medical conditions before embarking on a pregnancy journey.  

Lifestyle

Many modifiable lifestyle factors can increase the rate of preterm labor and birth, such as smoking, alcohol consumption, being over or underweight and drug abuse.

A too short or too long inter-pregnancy interval

The inter-pregnancy interval is defined as the time from the last delivery till the next pregnancy. Having kids too close or too far from one another can be associated with a higher rate of preterm birth. The optimal inter-pregnancy interval is around 18-23 months, and any pregnancy occurring outside this range is more likely to be complicated by many things including preterm birth.

Anatomic factors

Many women are born with congenital uterine malformations, such as unicornuate uterus, uterus didelphys, bicornuate uterus… and all of these variants can cause preterm delivery because the uterus is less likely to distend enough and accommodate a full term pregnancy. But keep in mind that such malformations are not contraindications for pregnancy and many pregnancies don’t become complicated, but I encourage you to have regular follow-ups with your physician in case of any known uterine malformation.

Moreover, surgeries done on the cervix (for the management of cancerous or precancerous cervical lesions) can cause cervical incompetence, which is defined by the inability of the cervix to hold a growing pregnancy. If you had any cervical surgeries before, inform your physician as proper counseling and high risk monitoring should be employed in that case.

Congenital malformations in the baby

Fetal congenital malformations occur in 3-4% of pregnancies and they can be as simple as a cleft lip or extra finger and as bad as major cardiac anomalies. Depending on the diagnosed malformation some babies are ought to be delivered prematurely for a postnatal intervention.

Placental dysfunction

Babies are always better off to stay in the womb as long as they are growing and developing. In other words, as long as the placenta is working properly and giving enough nutrients and oxygen to the baby, the latter would grow. When the pregnancy advances, the placenta ages progressively, but sometimes it starts aging and losing its function early on during pregnancy. This is reflected in a smaller than expected baby, a decreased amount of surrounding fluid, and some blood flow changes in the baby’s umbilical cord, liver and brain. When these conditions are observed, the baby would be better off outside the womb as the intrauterine condition is deteriorating, and this is when a medically indicated preterm delivery becomes warranted.

Mental health

Sometimes we focus on the bodily diseases and forget about mental health, but in fact, mental wellbeing is associated with a healthier and more prolonged pregnancy. Maternal clinical depression is linked to an earlier delivery or elective preterm termination of pregnancy.

So, there are many risks and reasons for a preterm delivery, and the closer to term the baby is born, the higher is the chance for an uncomplicated survival. Factors that have been associated with a better outcome in preterm babies are: gestational age at the time of delivery, birth weight, female sex (preterm female newborns do better than their male counterparts), level of neonatal intensive care unit (NICU) and expertise of the medical team, prenatal fetal condition (babies with pre-existing infections or congenital malformations do worse than otherwise healthy preterm babies), and ethnicity (black preterm babies have higher mortality rates than nonblack babies)

The goal of the physician is to predict, prevent and manage preterm labor and prolong pregnancy as much as medically possible. But what is your duty as a bearer?

The most important thing is for you to understand that working on the above mentioned modifiable risk factors is very beneficial in decreasing the rates of a preterm delivery. You may not be able to modify your age, ethnicity, uterine malformation or previous cervical surgery, but you are able to limit smoking, decrease alcohol consumption, avoid stress, lose the excessive weight or add the missing weight and to have routine health checks with your physician prior to and during pregnancy. All these steps will help in one way or another in limiting your risk for a preterm birth.
If you were deemed a high risk for preterm delivery, your doctor would be excepted to schedule more frequent follow ups and hospital visits, they would also regularly check on your cervical length by ultrasound, and might initiate medical or surgical interventions to minimize your risks (the interventions employed are beyond the scope of this article).

Moreover, never ignore labor symptoms no matter how early on they come, and no matter how many times you have been told “it’s not your time yet, you still have weeks to go”. Earlier diagnosis of preterm labor improves outcome. Any regular contractions, vaginal bleeding, fluid leakage, new intermittent back pain and pelvic pressure should prompt immediate investigation to rule out potential preterm labor. Yes, most of the times your symptoms are false alarms, but sometimes they will be true red flags, and you do not want to miss these few times. Never feel ashamed for frequently visiting the hospital, delivery ward or emergency room, and never feel sorry for calling your healthcare provider whenever you feel there is something wrong, because trust me, you know yourself the most. So, always listen to your body and seek professional help when in doubt.

In conclusion, prematurity is a serious condition, with millions of annual cases worldwide. Awareness campaigns should focus on educating women about the modifiable risks of preterm birth in order to limit its occurrence. Focus should be directed on underdeveloped countries which score the higher preterm birth rates with an associated high neonatal and infant mortality tolls. There are many success stories for extremely preterm neonatal survivals and NICU graduations, yet the journey is never easy on neither the newborn nor the parents nor the medical team!

Karam