What Week Is the Third Trimester of Pregnancy?

The third trimester of pregnancy begins at week 28 and continues through week 40, which is considered full term. That said, you may encounter sources that place the start at week 27, and the discrepancy reflects the fact that no universal medical body has drawn a hard line on the exact boundary. What everyone agrees on is that this final stretch brings the most dramatic fetal growth, the heaviest physical demands on the pregnant body, and a series of time-sensitive clinical screenings that can shape delivery outcomes.

Why the Start Week Varies Slightly

Most obstetric guidelines and professional organizations define the third trimester as beginning at 28 weeks of gestation. Some educational materials from hospitals and pregnancy apps round this to 27 weeks, partly because dividing 40 weeks into three equal trimesters lands the cutoff closer to 27 weeks and a few days. In clinical practice, the difference rarely matters. Providers track fetal development in weeks rather than trimesters, and the care schedule in this period is driven by specific gestational milestones rather than which trimester label applies.

One reason you might see conflicting numbers is that gestational age itself is an estimate. It is conventionally counted from the first day of the last menstrual period, which means it includes roughly two weeks before conception actually occurred. An early ultrasound scan is more accurate than menstrual dates at predicting the delivery date. Research has shown that in over half of cases, ultrasound-derived due dates are closer to the actual delivery date than those based on menstrual recall alone.¹ A study comparing early ultrasound, a smartphone app, and last menstrual period found that the ultrasound estimate fell within two weeks of the actual delivery date for about 88% of women, while the menstrual-period method only hit that window about 66% of the time.² Once a reliable early scan has been obtained, most clinicians base the entire timeline on that measurement.

What Happens to the Baby During This Stretch

The third trimester is when the fetus goes from roughly viable to ready for the outside world. At 28 weeks a baby weighs around two to two and a half pounds. By 40 weeks, average birth weight is close to seven and a half pounds. That weight gain is not just about getting bigger; it reflects the rapid accumulation of body fat that will help with temperature regulation and energy reserves after birth. Studies of fetal body composition confirm that subcutaneous fat measurements are significantly greater in normally growing fetuses compared to those with growth restriction, even as early as 30 to 31 weeks.³

Lung maturation is one of the most critical developments in the third trimester. The lungs begin producing surfactant, a substance that keeps the air sacs from collapsing with each breath, in meaningful quantities around weeks 26 to 28. Production ramps up steeply through weeks 34 to 36. This is the single biggest reason babies born very early face serious respiratory trouble, and it is also why steroid injections are given to mothers at risk of preterm delivery, to accelerate surfactant production.

The brain also undergoes a period of intense growth and folding during these final weeks. The cortex develops its characteristic grooves and ridges, the neural connections multiply rapidly, and the fetus begins cycling through recognizable sleep-wake patterns. By the late third trimester, a baby can respond to light, track sounds, and even show preferences for the mother’s voice. This brain development is a large part of why outcomes improve so dramatically with every additional week of gestation.

How the Mother’s Body Adapts

The cardiovascular system undergoes remarkable changes to support the growing fetus. Heart output and stroke volume peak around weeks 30 to 32, then gradually decline through the rest of the third trimester as the blood vessels tighten back up.⁴ Blood volume during pregnancy increases by roughly 40 to 50 percent overall, and the heart has to work harder to circulate all of it. This is why shortness of breath, swelling in the feet and ankles, and a noticeably faster resting heart rate are all common in the third trimester.

Musculoskeletal strain becomes much more pronounced as the uterus grows. The hormone relaxin, which loosens pelvic ligaments in preparation for delivery, also reduces the stability of the pelvis throughout the third trimester. Research on pelvic girdle pain shows that this loosening disturbs automatic movement patterns and measurably worsens balance, particularly during the second and third trimesters.⁵ The shift in the center of gravity, combined with ligament laxity, contributes to the characteristic gait changes and the increased fall risk that many pregnant women experience.

Sleep Position in Late Pregnancy

You have probably heard the advice to sleep on your left side during the third trimester. The reason is mechanical: the enlarged uterus compresses the inferior vena cava, the large vein that returns blood from the lower body to the heart, when you lie flat on your back. One study found that the supine position reduced heart output by about 16% compared to lying on the left side, while blood flow through the vena cava dropped dramatically, by as much as 85% at its origin.⁶ The body compensates by rerouting blood through smaller backup veins, but the overall circulation still takes a hit.

In practice, most people shift positions throughout the night without even knowing it. The key takeaway is to fall asleep on your side when you can, and not to panic if you wake up on your back. Some providers suggest placing a pillow behind your back to discourage fully flat positioning. The compression effect becomes more pronounced as the uterus grows heavier through weeks 28 to 40, so the advice applies more strongly in the later weeks of the trimester.

Screenings and Tests That Happen in the Third Trimester

Prenatal visits accelerate in the third trimester, typically moving from monthly to every two weeks around week 28 and then weekly from about week 36 onward. Several time-sensitive tests are clustered in this period.

One of the most important is the Group B Streptococcus (GBS) screen, usually performed between weeks 35 and 37. GBS is a common bacterium that can live harmlessly in the vaginal or rectal area but can cause serious infection in a newborn during delivery. A positive result means you will receive antibiotics during labor. Clinical guidelines recommend this screening window because it balances accuracy with proximity to delivery; the colonization status can fluctuate, and a screen done too early may not reflect what is present at the time of birth.⁷ Studies have confirmed that the 35-to-37-week window is the standard clinical practice across multiple countries.⁸

Growth monitoring also intensifies. Providers measure fundal height, the distance from the pubic bone to the top of the uterus, at each visit. If the measurement seems too large or too small for the gestational age, an ultrasound is ordered to estimate fetal weight more precisely. This combination approach has reasonably good accuracy for ruling out babies who are too small. Research shows that using fundal height as a trigger for targeted ultrasound yields a negative predictive value around 92%, meaning if the fundal height looks normal, there is roughly a 92% chance the baby is not abnormally small.⁹

Preeclampsia and Why It Matters Now

Preeclampsia, a condition marked by new-onset high blood pressure and protein in the urine, most commonly appears during the third trimester. It can progress rapidly to seizures, organ damage, or worse for both mother and baby.¹⁰ This is one reason blood pressure and urine checks happen at every third-trimester appointment. The condition is thought to originate in the placenta, though the exact cause is still debated. Risk factors include a first pregnancy, a history of the condition in a previous pregnancy, chronic high blood pressure, obesity, and carrying multiples.

The only definitive cure for preeclampsia is delivery, which puts providers in a difficult position when it appears well before term. Managing it involves balancing the risks of prematurity against the risks of allowing the condition to worsen. Mild cases may be monitored closely with more frequent visits, blood tests, and fetal heart rate monitoring. Severe cases often require hospitalization and may lead to early induction or cesarean delivery. If you notice sudden swelling in your face or hands, a persistent headache that does not respond to treatment, or visual disturbances during the third trimester, contact your provider promptly.

Braxton Hicks Contractions Versus the Real Thing

Braxton Hicks contractions, the irregular tightenings of the uterus that can start as early as the second trimester, tend to become more frequent and noticeable in the third trimester. They are sometimes called “practice contractions,” which is a bit misleading. Research shows that these contractions are actually asymmetric: they can involve just one region of the uterine wall rather than the coordinated, top-to-bottom squeeze that drives real labor. When a Braxton Hicks contraction involves the uterine wall away from the placenta, blood vessel resistance increases significantly and blood flow can even stop momentarily during certain phases. But when it involves the wall behind the placenta, blood flow is barely affected.¹¹

This localized nature is what distinguishes them from labor contractions, which engage the entire muscle in a coordinated wave from the top of the uterus downward. If contractions become regular, progressively stronger, and closer together, or if they are accompanied by fluid leaking or bleeding, that is a cue to call your provider. Before 37 weeks, regular contractions warrant particular urgency because of preterm labor concerns.

What “Term” Actually Means

The conventional definition of a term pregnancy is delivery between 37 weeks 0 days and 41 weeks 6 days.¹² But a workgroup of obstetric organizations recommended refining this into four subcategories because outcomes differ meaningfully even within that window:

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 distinction is not academic. Babies born at 37 weeks have somewhat higher rates of respiratory and feeding difficulties than those born at 39 or 40 weeks. That is why elective inductions and scheduled cesareans are now discouraged before 39 weeks unless there is a medical indication. The labeling shift from a single “term” category to these four tiers was designed to reduce unnecessary early deliveries that put babies at avoidable risk.

Viability and the Weeks Before the Third Trimester

Understanding when the third trimester starts also requires knowing what survival looks like if a baby arrives too early. At 22 weeks, survival among live-born infants is around 18%. By 24 weeks it reaches about 56%, and at 26 weeks it climbs to roughly 90%.¹³ Each additional week of gestation roughly triples the odds of survival for the most premature babies. Among those who do survive at the earliest gestational ages, over half experience a major complication during the newborn period. This steep survival curve is the backdrop against which the third trimester begins: week 28 marks a point where, if delivery were forced by complications, the baby’s chances are substantially better than they were just a few weeks earlier.

Vaccination Timing in the Third Trimester

The third trimester is also the recommended window for certain maternal vaccines, particularly the Tdap shot (tetanus, diphtheria, and pertussis). The timing is strategic. When a pregnant person receives Tdap, the immune system produces antibodies that cross the placenta and are present in the baby’s blood at birth. Research confirms that third-trimester Tdap vaccination is safe, allows efficient placental transfer of those antibodies, and does not interfere with the infant’s own immune response to pertussis vaccination later.¹⁴ Most guidelines recommend the shot between weeks 27 and 36, with earlier in that window being preferred so antibodies have more time to transfer before delivery.

RSV vaccination or monoclonal antibody timing for newborn protection follows a similar logic, and flu vaccination can be given at any point during pregnancy. The broader principle is that vaccines given during the third trimester take advantage of the placenta’s active antibody transport mechanism, which becomes most efficient in these final weeks.

Anxiety and Fear of Childbirth

The psychological landscape of the third trimester deserves mention because it is underappreciated in many pregnancy resources. A study of over 500 women in the third trimester found that more than half reported a meaningful level of fear about the upcoming delivery.¹⁵ That fear was associated with anxiety and depression, which suggests it is not simply normal nervousness but something that can shade into clinical territory. Excessive screen time, particularly more than five hours a day, was also linked to higher fear levels, possibly through exposure to dramatic or negative birth stories online.

If you find that worry about labor is dominating your thoughts, disrupting your sleep, or making it hard to enjoy the pregnancy, bringing it up with your provider can open the door to useful interventions. Childbirth education classes, conversations with experienced doulas, and cognitive-behavioral approaches have all shown benefit. The third trimester is also when birth plans are typically discussed, and having a sense of agency in the process can reduce some of the anticipatory dread.

Why Human Pregnancy Lasts as Long as It Does

There is a longstanding idea that human babies are born “too early” compared to other primates because the pelvis is only barely wide enough for a newborn’s head. This so-called obstetric dilemma posits that bipedal walking constrained pelvis size, forcing birth to happen before the brain grows too large to fit. More recent research challenges this narrative. An analysis of metabolic data across species suggests that the real constraint on human pregnancy length is the mother’s metabolic ceiling, not her pelvis. In other words, human gestation ends around 40 weeks because the energy demands of the fetus begin to exceed what the mother can sustain, regardless of head size.¹⁶ The baby’s caloric needs in the final weeks of the third trimester are enormous, roughly doubling between weeks 30 and 40, and the mother’s metabolic rate approaches a hard biological limit. Birth is less about squeezing through the pelvis and more about reaching the outer boundary of what the body can fuel.