The 4 Critical Stages Of Term Pregnancy: Why ACOG’s 39-Week Rule Is A Game-Changer

Jan 10, 2026 • minute read

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For decades, the term "full term" was a broad, five-week window that often led to confusion and unnecessary early deliveries. However, in a major update to medical guidelines, the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) have redefined what it truly means for a baby to be "term." This change, based on extensive research into fetal development, is one of the most significant updates in modern obstetrics, emphasizing that every week counts right up to the end of your pregnancy journey.

As of today, December 20, 2025, the medical community no longer considers a baby born at 37 weeks to be "full term." Instead, the new guidelines—designed to reduce neonatal morbidity and improve infant health—break the final weeks of gestation into four distinct, critical stages. Understanding these stages is essential for expectant parents and their healthcare providers to ensure the best possible start for the baby.

The ACOG/NICHD Revised Definition: Four Stages of Term Birth

The previous, outdated definition of "term" was a wide range spanning from 37 weeks to 42 weeks. The new classification, based on research from the National Institute of Child Health and Human Development (NICHD), provides a more precise and medically relevant framework. This precision highlights the subtle, yet critical, differences in health outcomes for infants born across this five-week period.

Here is a detailed breakdown of the four official categories of term birth, which replace the single, ambiguous "term" label:

Early Term: 37 weeks 0 days through 38 weeks 6 days
Full Term: 39 weeks 0 days through 40 weeks 6 days
Late Term: 41 weeks 0 days through 41 weeks 6 days
Post Term: 42 weeks 0 days and beyond

The key takeaway is that Full Term is now specifically defined as 39 weeks through 40 weeks and 6 days. This is the optimal window for delivery in a healthy, uncomplicated pregnancy, as it offers the lowest risk of adverse neonatal outcomes.

Why Weeks Matter: The Critical Development Between 37 and 39 Weeks

The most compelling reason for the ACOG's redefinition was the overwhelming evidence that babies born in the "Early Term" window (37-38 weeks) face significantly higher health risks compared to those born at 39 weeks or later. This is often summarized by medical professionals with the phrase, "Weeks matter."

The final weeks of gestation are not merely a period of weight gain; they are a crucial time for the maturation of vital organ systems. Major developmental milestones are completed during this period:

1. Lung Maturation and Breathing

One of the most significant differences between an Early Term and a Full Term baby is lung maturity. While a baby's lungs are functional at 37 weeks, they are still "ripening" and developing the necessary surfactant—a substance that keeps the air sacs from collapsing. Babies born at 37 or 38 weeks are at a higher risk of developing Respiratory Distress Syndrome (RDS) and transient tachypnea of the newborn (TTN), which are breathing problems requiring medical intervention.

2. Brain and Nervous System Development

The brain continues to undergo rapid and crucial growth between 37 and 39 weeks. Studies indicate that the brain of a baby at 39 weeks weighs about a third more than the brain of a baby at 35 weeks. This final period allows for essential connections to form, crucial for long-term health, learning, and coordination. The extra time helps with temperature regulation, feeding coordination, and overall neurological stability.

3. Liver and Body Fat Reserves

The liver, which is responsible for processing bilirubin, also matures in these final weeks. Early Term babies have a higher risk of jaundice because their livers are not fully ready to process bilirubin efficiently. Additionally, the accumulation of subcutaneous fat during this time is vital for maintaining a stable body temperature after birth, reducing the risk of hypothermia and hypoglycemia (low blood sugar).

Risks Associated with Non-Full Term Deliveries

While a spontaneous delivery at 37 or 38 weeks can be perfectly healthy, the new guidelines were specifically created to discourage elective, provider-initiated deliveries (like scheduled C-sections or inductions) before 39 weeks without a compelling medical indication. The data clearly shows that the risk of neonatal morbidity is significantly lower once the baby reaches the 39-week mark.

Risks of Early Term Birth (37w0d to 38w6d)

Babies in this category have an increased risk of several adverse outcomes compared to those born Full Term. These risks include:

Higher rates of Neonatal Intensive Care Unit (NICU) admission.
Respiratory complications (RDS, TTN).
Difficulty maintaining body temperature (Hypothermia).
Feeding difficulties and low blood sugar (Hypoglycemia).
Developmental delays later in childhood, though subtle.

Risks of Late Term and Post Term Birth (41 Weeks and Beyond)

While the focus is often on early delivery, going too far past the Estimated Date of Delivery (EDD)—the 40-week mark—also introduces its own set of risks for both mother and baby.

Placental Insufficiency: The placenta may start to age and become less efficient at delivering oxygen and nutrients.
Macrosomia: The baby continues to grow, increasing the risk of a difficult vaginal delivery, shoulder dystocia, and birth trauma.
Meconium Aspiration: The baby may pass its first stool (meconium) in utero, which can lead to serious breathing problems if inhaled.
Maternal Risks: Increased risk of infection and the need for a Cesarean section (C-section).

For this reason, a pregnancy that reaches 41 weeks (Late Term) is often closely monitored, and healthcare providers frequently discuss induction options to prevent the pregnancy from becoming Post Term (42 weeks or more).

Key Entities and Terminology in Term Pregnancy

To achieve high topical authority on this subject, it is important to be familiar with the core medical entities and acronyms used by clinicians and researchers:

ACOG: American College of Obstetricians and Gynecologists. The leading professional organization that sets the standards for obstetrical care in the U.S.
NICHD: National Institute of Child Health and Human Development. The research body whose studies informed the new term definitions.
SMFM: Society for Maternal-Fetal Medicine. Specialists in high-risk pregnancy who collaborated on the new guidelines.
Gestational Age: The age of the pregnancy measured from the first day of the mother’s last menstrual period (LMP), typically expressed in weeks and days.
EDD (Estimated Date of Delivery): The "due date," calculated as 40 weeks from the LMP.
Preterm Birth: Delivery before 37 weeks 0 days gestation.
Neonatal Morbidity: Health problems or diseases experienced by a newborn infant. The goal of the new definition is to reduce the risk of this.
Respiratory Distress Syndrome (RDS): A breathing problem in newborns, especially premature infants, caused by a lack of surfactant in the lungs.
Hypoglycemia: Abnormally low blood sugar in the newborn, a common risk for Early Term babies due to immature liver function.
Induction of Labor: The use of medication or other methods to start labor artificially.
Placenta: The organ that develops in the uterus during pregnancy to provide oxygen and nutrients to the baby.

The shift in the definition of "full term pregnancy" represents a major advancement in maternal and fetal health. It serves as a clear, evidence-based guideline for both providers and parents: in the absence of a medical necessity, aiming for a delivery within the Full Term window of 39 weeks 0 days to 40 weeks 6 days offers the baby the best possible health outcomes. The final weeks of gestation are a critical developmental period that should be protected to ensure the baby's brain, lungs, and liver are fully mature for life outside the womb.

The 4 Critical Stages of Term Pregnancy: Why ACOG’s 39-Week Rule is a Game-Changer

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