A baby spends months developing lungs that are not yet being used to breathe air. Inside the womb, oxygen reaches the baby through the placenta and carbon dioxide returns through the placental circulation. The fetal lungs remain fluid-filled while preparing for a job they will fully take over only after birth. Calling antenatal corticosteroids “lung-maturity injections” is useful shorthand, but it can create the impression that one course somehow completes lung development.
The useful question is: If antenatal steroids do not make the baby breathe inside the womb, what exactly are they preparing the lungs to do after birth?
Before Birth, the Placenta Does the Gas Exchange
Fetal breathing movements do occur, but they are movements of a fluid-filled lung rather than breaths of air, and they do not provide oxygen to the baby. Before birth, the placenta performs gas exchange and pulmonary vascular resistance remains high, so pulmonary blood flow is much lower than it will be after birth. [2]
At birth, that arrangement must change quickly. Air must enter the lungs, fetal lung liquid must move out of the air spaces, surfactant must help stabilise the newly aerated lung, pulmonary vascular resistance must fall and pulmonary blood flow must rise. The lungs can then take over oxygen and carbon-dioxide exchange. [2]
Lung Maturity Is a Continuum, Not a Switch
Human lung development occurs in overlapping stages rather than at rigid cut-off dates. During the canalicular period, roughly 16 to 26 weeks, respiratory bronchioles and early gas-exchange structures develop while specialised epithelial cells and the pulmonary capillary network become increasingly organised. During the overlapping saccular period, beginning at about 24 weeks and continuing through late gestation, terminal air spaces enlarge, the tissue barrier becomes thinner and the surfactant system continues to mature. Alveolar development begins in late fetal life and continues after birth. [1]
This is why “lung maturity” is not an on-or-off event. A very preterm lung after antenatal steroids is still a very preterm lung. Additional gestational time continues to add structural, cellular and vascular development.
Steroids Accelerate Maturation - They Do Not Replace Time
Antenatal corticosteroids cross the placenta and accelerate selected maturational processes in the fetal lung. Their effects include maturation of the surfactant system, epithelial differentiation and changes involved in pulmonary fluid handling. [3]
Surfactant lowers surface tension after the lung becomes air-filled, helping the tiny distal air spaces remain open between breaths. Antenatal steroids can therefore improve an immature lung's readiness for the transition at birth, but they do not instantly complete structural lung development or turn a preterm lung into a term lung. [2,3]
Whether antenatal corticosteroids are appropriate in an individual pregnancy depends on gestational age, the likelihood and timing of preterm birth, previous steroid exposure and the complete maternal-fetal clinical situation. The decision should not be made simply from an embryological stage or from the general idea that the baby's lungs need “maturing.” [3]
Dr Tania’s Clinical Insight: Antenatal steroids do not “finish” fetal lung development. They accelerate selected maturational changes that help an immature, fluid-filled fetal lung prepare for the transition to air breathing after birth. A steroid course can improve readiness; it cannot replace the structural, vascular and alveolar development that comes with gestational time.
Evidence Base
[1] Rivera-Sanchez Y, Blair ZW, Wall TJ, Goss KN. Growing Up With Developmental Lung Diseases: A Review for the Adult Pulmonologist. Chest. 2025;168(1):95–105. doi:10.1016/j.chest.2025.02.014.
[2] Sankaran D, Lakshminrusimha S, Saugstad OD. Physiology of neonatal resuscitation: Giant strides with small breaths. Seminars in Perinatology. 2022;46(6):151620. doi:10.1016/j.semperi.2022.151620.
[3] Tan Y, Stark MJ, Clifton VL, Wiese MD, Morrison JL, Gatford KL. Glucocorticoids in pregnancy: A master-switch for fetal maturation. Endocrine Reviews. Published online May 23, 2026. doi:10.1210/endrev/bnag015.
Medical Disclaimer
This Clinical Insight provides general education and does not determine whether antenatal corticosteroids are indicated in an individual pregnancy. The decision depends on gestational age, the likelihood and timing of preterm birth, previous treatment and the complete maternal-fetal clinical picture. Do not request, repeat, delay or decline prescribed antenatal corticosteroids because of information on this page. Seek prompt maternity assessment for regular painful contractions, leaking fluid, vaginal bleeding, reduced fetal movements or any concern that preterm birth may be occurring.