Determination of Fetal Lung Maturity

Introduction:

Babies receive oxygen from the mother's bloodstream through the placenta when in the womb. Though the baby's lungs start developing as early as four weeks of gestation, they take their first breath only after birth. However, the complex respiratory system keeps maturing and is completely formed by 36 weeks of gestation. The pulmonary system is the last to mature among all the systems, both structurally and functionally. Fetal sex plays a role in lung maturity, with respiratory problems being more common in male fetuses when compared to female fetuses.

Determination of fetal lung maturity is essential, especially in prematurely born babies. In babies born between 24 and 35 weeks of gestation, doctors give a medicine called steroids that helps in lung maturation. In addition, in cases where preterm birth is preplanned due to inevitable factors, specific laboratory tests performed on the amniotic fluid help assess fetal lung maturity, which is essential for the fetus's survival.

What Are the Phases of Fetal Lung Maturity?

There are five major phases of fetal lung maturity, which include:

• Embryonic Phase: The embryonic phase starts around four to five weeks of gestation. During this stage, two tiny buds branch off, out of which one forms the future right lung and the other the future left lung. The larynx (voice box) and trachea (windpipe) develop from the foregut.

• Pseudoglandular Phase: The pseudo glandular phase is between five to seventeen weeks of gestation. During this phase, the lung buds branch into numerous small units. Over time, each bud develops into an independent respiratory unit with bronchioles and capillaries.

• Canalicular Phase: The canalicular phase is between 16 to 24 weeks of gestation. During this phase, a barrier develops between the blood and the air. Different types of tissues develop in the lungs during this phase.

• Saccular Phase: Saccular phase starts at the 26th week of gestation. During this phase, the surfactant forms. Surfactant is a fluid that prevents damage to delicate lung tissue. It allows the amniotic fluid to drain away and fill the lungs with air after delivery. Premature babies are at risk of lung collapse due to a lack of sufficient surfactants.

What Is Practice Breathing?

Even though babies do not breathe till they are born, they start taking practice breaths as early as ten weeks of gestation by inhaling and exhaling small amounts of amniotic fluid. This process is vital for normal lung development.

By 28 weeks, a gas exchange may start in the tiny lungs, even though the alveoli (tiny sacs in the lungs that help exchange oxygen and carbon dioxide) and surfactant are not fully formed. By 35 weeks of pregnancy, the fetus may have enough surfactant that can enable them to breathe outside the uterus. At 36 weeks, the baby's lungs are completely formed, and by 40 weeks, the baby has about 150 million alveoli ready to help the baby breathe on their own after birth.

In cases where a baby is born early or has difficulty breathing can require a few hours in NICU (neonatal intensive care unit). Babies born preterm require a more extended period in a NICU and surfactant therapy. Some may even face long-term respiratory problems.

Why Is Fetal Lung Maturity Important in Premature Babies?

The baby's internal organs develop during the first three months (first trimester) of gestation. However, they continue to mature in the second and third trimesters. Babies born before 34 weeks of gestation can have difficulty in respiration as the lungs do not mature completely and allow the exchange of gases.

Due to advances in the medical field, fetal lung maturity can be achieved through medicines. This is called pharmaceutical lung maturation. This is achieved by administering intramuscular steroid injections to the mother, which helps mature the fetus's lungs. The treatment includes administering two injections given 24 hours apart. Fetal lungs mature after administration of both doses. In emergency premature C-sections, the doctor ensures at least one dose of the steroid injection.

What Are the Criteria to Be Considered Before Initiating Fetal Lung Maturation?

Though fetal lung maturity may be vital in the case of preterm babies, the doctors must consider the baby's gestational age and any counter-indications that the mother might have. For example, pharmaceutical lung maturation is dangerous for mothers with complications like gestational diabetes or arterial hypertension. Initiating fetal lung maturity in such cases can affect the mother's health. Instead, constant fetal monitoring in place of steroid injections must be considered. However, in cases with no risks to the mother, administering steroids to attain fetal lung maturity can help save the baby's life.

When Is Fetal Lung Maturity Testing Done?

Information about lung maturity may be helpful; however, these assessments need to be clarified further. The test results help assess the level of care a newborn may require after birth. This information also helps assess the balance between maternal and fetal risks of preterm delivery or continuation of pregnancy.

Fetal lung maturity is no longer determined in the case of preterm delivery to aid in the delivery timing. However, owing to the risks to the mother or fetus, if a preterm delivery has to be initiated, it must be performed. Also, fetal lung maturity testing does not determine the maturity status of other fetal organs.

How Commonly Is Fetal Lung Maturity Measured?

• Fetal lung maturity testing, also known as FLM testing, is rarely done because it is believed that the test cannot assess the need for preterm delivery completely.

• Determination of lung maturity through FLM testing does not indicate the maturity of other organs.

• If there is a clear indication for preterm delivery due to the mother's or fetus' condition, it must be carried out irrespective of the lung maturity status.

• Recently, guidelines have changed regarding the initiation of preterm delivery, and fetal lung maturity is not the only criterion to consider in the decision.

• Studies show there have been worse outcomes after preterm births resulting in fetal complications.

• However, the fetal lung maturity was rated as immature, borderline, or mature based on the measured phospholipid levels.

• The FLM results can vary depending on the gestational age and blood or meconium-stained amniotic fluid.

• Current guidelines are based on measuring other maternal and prenatal metrics rather than laboratory tests like FLM testing.

Conclusion:

Fetal lung maturity determination was one of the important criteria to proceed with the decision of preterm births. However, it is not the only factor being considered in the recent past. Whatever the testing, the ultimate goal of the clinician is the safe delivery of a preterm infant and enabling conditions that help best in its survival outside the womb.