Week 27 lands at the very tail end of the second trimester under most medical guidelines, with the third trimester officially kicking off at week 28. A smaller group of clinicians begins counting the third trimester at week 27 because fetal lung and brain development markers shift meaningfully at that point. The exact answer depends on your provider’s protocol, your due date, and which milestones matter most for your prenatal schedule.
Trimester boundaries can feel blurry when you’re counting down the weeks, especially right around the 27-week mark, so here’s what to know as you near the home stretch of pregnancy.
How Pregnancy Is Divided Into Three Trimesters
A typical pregnancy stretches across roughly 40 weeks from the first day of your last menstrual period, dividing into three roughly equal phases of about 13 weeks each. The American College of Obstetricians and Gynecologists treats these as approximate phases rather than rigid cutoffs, which is why the exact boundary between one phase and the next can shift depending on who’s counting.
The first trimester runs from week 1 through week 13, the second from week 14 through week 27, and the third from week 28 until delivery around week 40. Because 40 doesn’t split cleanly into thirds, one phase carries an extra week, and most guidelines tuck that week into the second trimester.
The Standard Week Ranges
| Trimester | Week Range | Focus |
|---|---|---|
| First | Weeks 1–13 | Organ formation, early screening |
| Second | Weeks 14–27 | Growth, movement, anatomy scan |
| Third | Weeks 28–40 | Final maturation, labor prep |
The second trimester is often called the “honeymoon phase” because energy returns, morning sickness fades, and your baby’s movements become noticeable. Once that window closes, the body shifts its priorities toward delivery prep.
Where Week 27 Actually Falls in the Timeline
Under the most common classification, week 27 sits in the final days of the second trimester. The third trimester begins at week 28, so you’re still inside phase two when you hit week 27, even though you’re one week from the boundary. Some clinicians shift the line back to week 27 because fetal development markers at that point begin to mirror what’s expected in the third phase.
Due date calculation becomes a sticking point here. Pregnancy weeks are counted from the first day of your last menstrual period, not from conception, so two people with the same conception date can land on different week numbers depending on cycle length. If your provider adjusts your due date after an early ultrasound, the trimester boundary shifts with it.
Practical tip: ask your provider which week range they use to define the third trimester. Most follow the week 28 start, but some use week 27 based on fetal lung and brain development markers. Your prenatal visit schedule and screening milestones both depend on that answer.
The clinical relevance is real. Prenatal visit frequency often increases once you cross into week 28, and certain screenings get scheduled based on gestational age rather than trimester label. Knowing exactly where you sit helps you anticipate the next checkpoint.
Fetal Development Milestones at Week 27
By week 27, the fetus weighs roughly 2 pounds and measures about 14 inches from head to heel, roughly the size of a cauliflower. The lungs begin producing surfactant, a substance that keeps air sacs open after birth, and the brain now shows recognizable patterns of activity tied to sleep and wake cycles. Eyes open and close in response to light and dark, and the nervous system is wiring up at a fast clip.
Fetal viability improves sharply between weeks 24 and 28, especially with modern neonatal intensive care. Viability here means the point at which a baby born prematurely has a meaningful chance of surviving outside the womb with medical support. Reaching week 27 is a real milestone because outcomes continue to improve week by week from here.
What’s New This Week
- Surfactant production: the lungs begin making the substance needed for breathing air after birth.
- Brain wave patterns: activity cycles between sleep and wake states you might notice in movement patterns.
- Sensory response: eyes open and close in reaction to light filtering through the uterine wall.
- Fat accumulation: the fetus lays down brown and white fat to regulate body temperature after delivery.
These changes happen fast. A week-by-week difference at this stage is meaningful in terms of lung maturity and overall readiness, which is why providers watch the timeline closely once you cross into the late second trimester.
Common Symptoms and Body Changes Around This Stage
Fatigue often makes a comeback around week 27 because blood volume has increased by roughly 40–50% and your body is carrying significantly more weight. Lower back pain and pelvic pressure intensify as the hormone relaxin loosens joints in preparation for delivery. Braxton Hicks contractions, which are practice contractions that come and go without a regular pattern, become more frequent and easier to notice, especially after physical activity or at the end of the day.
Swelling in the feet, ankles, and hands is common by this stage, and shortness of breath shows up as the uterus pushes up against the diaphragm. None of this means something is wrong, but each symptom is worth tracking so you can describe changes to your provider with precision.
Signs That Deserve a Call to Your Provider
- Severe headache: especially with vision changes or upper abdominal pain, which can signal a blood pressure problem.
- Sudden swelling: rapid puffiness in the face, hands, or feet rather than the gradual ankle swelling that’s typical.
- Heavy bleeding: any bright red bleeding, especially with cramping.
- Leaking fluid: a steady trickle or gush that doesn’t stop, which may mean ruptured membranes.
- Reduced fetal movement: noticeably fewer kicks or rolls over a full day once kick counts are part of your routine.
Most discomfort at week 27 is normal, but your provider would rather hear from you than have you wait. Trust your instincts and call when something feels off.
How Prenatal Care Routines Shift in the Final Phase
Once you cross into week 28, prenatal visits typically move from monthly to every two weeks, and then to weekly after week 36. Standard checks include blood pressure, urine protein screening, fundal height measurement (the distance from your pubic bone to the top of your uterus), and fetal heart rate monitoring. These visits also give you space to ask questions and flag symptoms.
Glucose screening for gestational diabetes is usually completed between weeks 24 and 28, so that test is either done or about to be done. If your results came back normal, you’re in the clear. If they came back borderline, your provider will discuss next steps and follow-up testing.
What to Bring Up at Your Next Visit
- Birth plan basics: preferences for labor, delivery location, and support people.
- Work and leave timing: when you plan to stop working and how to document any workplace accommodations.
- Pediatrician selection: choosing a provider for your baby before delivery makes the first days smoother.
- Feeding plan: whether you plan to breastfeed, formula feed, or combine, and what support you’ll want.
- Warning signs: a clear list of symptoms that mean “call right now” versus “mention at the next appointment.”
These conversations go faster when you’ve thought about them beforehand. Jot down questions between visits so nothing important gets lost in the moment.
Practical Ways to Prepare for the Third Trimester
The last 13 weeks go faster than expected, and small steps now save stress later. Stock postpartum supplies and newborn essentials early, because shopping at 36 weeks is harder than it sounds. Begin light pelvic floor exercises with your provider’s approval, since a strong pelvic floor supports recovery after delivery.
Plan maternity leave logistics before you’re tired enough that paperwork feels impossible. Track fetal movement patterns once your provider asks for daily kick counts, usually starting around week 28. Schedule a hospital tour and finalize your pediatrician so the final stretch runs on autopilot.
A Short Prep Checklist
- Set up the sleep space: crib or bassinet assembled, sheets washed, room layout planned.
- Stock postpartum basics: pads, comfortable clothing, perineal care items, and any nursing supplies.
- Prep freezer meals: a few slow-cooker or oven-ready dishes cover the first busy week home.
- Confirm transportation: know the route to your delivery location and have a backup plan.
- Build a support list: friends or family who can help with meals, errands, or older siblings.
Done early, these tasks stay small. Done late, they crowd the same week you’re trying to rest.
Bottom Line
Week 27 sits at the very end of the second trimester under standard guidelines, but the third trimester begins just one week later, and a few clinicians start counting it from week 27 based on fetal milestones. You’re in the final stretch of the second phase, and the next few weeks bring measurable changes for your baby and your body. Treat this window as the right time to finalize plans, stock supplies, and shift prenatal visits into the next rhythm.
FAQ
Is 27 weeks considered the third trimester?
Most major medical guidelines place week 27 squarely in the second trimester, not the third. The third trimester officially begins at week 28. Some clinicians start counting the third trimester at week 27 because of fetal development markers, so the answer depends on the protocol your provider follows.
What trimester is 27 weeks in?
Standard classification puts week 27 as the final stretch of the second trimester. You’re entering the home stretch of phase two, and your provider will likely shift to third-trimester care routines within the next week or two.
When does the third trimester officially start?
Week 28 is when most guidelines flip the switch and mark the official start of the third trimester. ACOG describes the trimesters as three roughly equal phases, and the math puts the third-phase boundary at week 28 for a 40-week pregnancy.
How many weeks are in each trimester of pregnancy?
Each phase runs about 13 weeks from start to finish. The first covers weeks 1–13, the second covers weeks 14–27, and the third covers weeks 28–40, since 40 doesn’t divide evenly into thirds.
How many weeks are in the third trimester of pregnancy?
From week 28 through delivery around week 40, the third trimester covers about 13 weeks of pregnancy. Full-term delivery happens between weeks 39 and 40, while preterm birth is classified as delivery before week 37.
What happens to the baby at 27 weeks of pregnancy?
The fetus weighs about 2 pounds, measures around 14 inches, and is producing surfactant in the lungs in preparation for breathing air. Brain wave activity shows recognizable sleep and wake cycles, and the eyes open and close in response to light and dark.
