Find out exactly when the third trimester starts and what to expect week by week — symptoms, milestones, and how to prepare for birth and beyond.
June 5, 2026·16 min read
There is something that shifts when you enter the third trimester. The pregnancy that once felt distant and abstract becomes suddenly, undeniably close. The baby you have been growing for months is now the size of a pineapple, a squash, a small watermelon — and the countdown to meeting them has officially begun.
If you are searching for when the third trimester starts, you are probably either approaching it and want to know exactly when you cross that threshold, or you are already there and trying to understand what lies ahead. Either way, you are in the right place — and the good news is that the third trimester, while demanding, is also one of the most meaningful stretches of the entire pregnancy journey.
This guide covers exactly when the third trimester begins, how the weeks unfold from 28 to 40 and beyond, what physical and emotional changes to expect, how your baby is growing, and how to prepare yourself for labor, birth, and the first days of life on the other side. Nothing is glossed over. Nothing is exaggerated. Just the real, week-by-week picture of what this final trimester holds.
When Does the Third Trimester Start?
The third trimester begins at week 28 of pregnancy — measured from the first day of your last menstrual period (LMP), which is the standard way pregnancy is dated.
Here is how the three trimesters break down:
- First trimester: Weeks 1–13
- Second trimester: Weeks 14–27
- Third trimester: Weeks 28–40 (and beyond, if pregnancy continues past the due date)
You may occasionally see slightly different definitions — some sources place the start of the third trimester at week 27, others at 28. The week 28 marker is the most widely used in clinical obstetric practice and is the definition most care providers follow.
At 28 weeks, your baby is approximately 14–15 inches long and weighs roughly 2.5 pounds. The lungs are developing rapidly, the eyes can open and close, and the baby has established sleep and wake cycles. Babies born at 28 weeks — while still premature — have significantly improved survival rates compared to earlier preterm births, which is one reason this milestone carries clinical significance beyond simply marking a trimester.
The third trimester spans approximately 12 weeks — the shortest of the three trimesters by definition, though it rarely feels that way from the inside.
What Happens in Your Body as the Third Trimester Begins
The transition from second to third trimester brings a noticeable shift in how pregnancy feels physically. The relative comfort many women experience in the second trimester — the "honeymoon phase" of pregnancy, when nausea has often passed and the belly is not yet fully demanding — begins to give way to the more significant physical demands of the final stretch.
The belly grows significantly. From week 28 onward, the uterus is expanding rapidly to accommodate a baby who gains approximately half a pound per week in the final months. This growth affects everything: posture, breathing, sleep position, digestion, and how far you can comfortably walk or stand.
The center of gravity shifts. As the belly moves forward and downward, the lower back compensates by curving inward. This shift is the primary cause of the lower back pain that becomes common in the third trimester — the body is doing its best to stay balanced against a weight distribution it was not designed to carry indefinitely.
Braxton Hicks contractions become more noticeable. Practice contractions — the tightening and releasing of the uterine muscle — become more frequent and more noticeable from around week 28 onward. They are typically painless or mildly uncomfortable, irregular, and do not progress. They are the uterus rehearsing for labor.
Sleep becomes more challenging. Between a growing belly that limits comfortable positions, frequent urination, Braxton Hicks, leg cramps, and the general restlessness of late pregnancy, the third trimester is often marked by significantly disrupted sleep. This is difficult and worth taking seriously — rest matters, and asking for help to protect it is reasonable.
Shortness of breath may begin or worsen. As the uterus grows upward and presses against the diaphragm, lung capacity is temporarily reduced. Many women find that activities that were previously easy — climbing stairs, walking quickly, talking while moving — leave them more breathless than expected. This typically improves somewhat when the baby drops (called lightening) in the final weeks.
Week-by-Week Guide to the Third Trimester
Week 28: The Third Trimester Begins
Welcome to the third trimester. At 28 weeks, your baby's brain is developing rapidly, the eyes open and respond to light, and the lungs are producing surfactant — the substance that will allow them to breathe independently after birth. If your provider has not already discussed it, this is typically when the RhoGAM injection is offered to Rh-negative mothers, and when glucose tolerance testing results (done around weeks 24–28) are reviewed. Kick counts — tracking your baby's movements — become a routine recommendation from this point onward.
Week 29
Growth is accelerating. Your baby is adding layers of fat beneath the skin, giving them the rounder appearance of a full-term newborn. Muscle and lung development continue at pace. For you, heartburn often intensifies around this week as the uterus pushes further into the stomach's territory. Eating smaller, more frequent meals and staying upright for at least an hour after eating can help.
Week 30
At 30 weeks, your baby is approximately 15–16 inches long and nearing 3 pounds. Their bone marrow has now taken over red blood cell production entirely. For many mothers, week 30 brings a psychological shift — double digits until the due date, and the reality of preparation begins to feel genuinely urgent. This is a good week to start thinking about your birth plan if you have not already.
Week 31
The baby's brain connections are multiplying at a remarkable rate. Hearing is well developed and babies at this stage can recognize familiar voices — particularly yours. Swallowing, blinking, and breathing practice movements are regular. For you, the added weight may begin to affect your hips and pelvis; pelvic girdle pain or round ligament pain are common complaints from this point onward. Gentle prenatal yoga or swimming can help significantly.
Week 32
At 32 weeks, most babies are beginning to move into a head-down position in preparation for birth — though there is still time and room to shift. Toenails and fingernails are fully formed. For you, this is typically when prenatal appointments shift to every two weeks rather than monthly. Your care provider will begin tracking the baby's position at each visit. If you have not yet toured the hospital or birth center, now is a good time.
Week 33
The baby's immune system is developing rapidly, receiving antibodies through the placenta. Bones are hardening, though the skull remains flexible to allow passage through the birth canal. Braxton Hicks contractions may feel more intense and frequent this week. Many women report an increase in vivid dreams and pregnancy-related anxiety around weeks 33–35 — this is extremely common and worth discussing with your care provider if it is affecting your sleep or daily functioning.
Week 34
At 34 weeks, your baby's lungs are maturing significantly. Babies born at 34 weeks typically do well with some medical support, though full-term is still the goal. The Group B Streptococcus (GBS) swab test is typically done between weeks 35 and 37 — your provider will discuss this at upcoming visits. For you, swelling in the feet and ankles becomes more common from this point, particularly at the end of the day. Elevating your feet, staying hydrated, and reducing salt can help.
Week 35
This week marks an important milestone: babies born at 35 weeks are generally considered late preterm and have much better outcomes than earlier preterm births. The baby is now gaining weight rapidly — roughly half a pound per week — and the body is becoming properly rounded. Your hospital bag should ideally be packed by the end of this week. Labor can and does begin before 40 weeks, and being prepared earlier rather than later removes one significant source of stress.
Week 36
At 36 weeks, your prenatal appointments typically shift to weekly visits. The baby is almost fully developed — the primary work between now and birth is weight gain and lung maturation. Many babies begin to drop lower into the pelvis around this time, a process called lightening or engagement. You may notice it becomes easier to breathe as the baby moves away from the diaphragm, while pelvic pressure and frequent urination increase. A 36-week pregnancy is considered late preterm; from 37 weeks onward it is considered early term.
Week 37
Week 37 is the beginning of early term pregnancy. A baby born this week is generally healthy but may still benefit from additional time for brain, lung, and liver development. The World Health Organization and most obstetric bodies currently recommend allowing labor to begin naturally unless there is a medical indication for earlier delivery. Your care provider may begin discussing cervical checks and signs of early labor from this point.
Week 38
The baby is considered full term at 38 weeks by most definitions, though the optimal range extends to 40 weeks and beyond. Vernix — the waxy coating that has protected the baby's skin in the amniotic fluid — is beginning to reduce. Lanugo, the fine hair that covered the body, is mostly shed. Many women feel a burst of nesting energy around this week — the urge to clean, organize, and prepare everything becomes almost compulsive for some. Honor it.
Week 39
At 39 weeks, the baby's brain, lungs, and liver are continuing to develop even now. Research consistently shows that babies allowed to reach 39–40 weeks have better outcomes across multiple measures compared to those delivered at 37–38 weeks without medical indication. For you, the discomfort of late pregnancy may be reaching its peak — difficulty sleeping, pressure, Braxton Hicks, and the emotional weight of waiting. This is a good week to discuss any outstanding birth plan questions with your provider.
Week 40: Your Due Date
Your estimated due date arrives at 40 weeks — but it is important to understand that a due date is an estimate, not a deadline. Only approximately 5% of babies are born on their actual due date. The normal range for full-term delivery extends from 37 to 42 weeks. Many first-time mothers go past their due date; this is common and generally not concerning in uncomplicated pregnancies.
At week 40, your care provider will typically begin discussing your options if labor has not started — monitoring, membrane sweeping, and induction planning if the pregnancy continues past 41–42 weeks.
Week 41 and Beyond
Going past your due date is frustrating, but it is also common — particularly for first-time mothers, who on average deliver at 41 weeks and 1 day. Your provider will increase monitoring frequency and discuss the relative benefits and risks of waiting versus induction based on your specific circumstances. Most providers recommend induction by 42 weeks, and many offer it between 41 and 41+6 depending on guidelines and individual factors.
The third trimester is not the final chapter — it is the last preparation. Everything your body is doing in these weeks is getting both of you ready for the most important moment of your lives so far.
Baby Development in the Third Trimester: Key Milestones
While the week-by-week guide covers development in context, here is a consolidated view of the major milestones your baby reaches in the third trimester:
Weeks 28–30: Rapid brain development; eyes open; surfactant production begins; bone marrow takes over red blood cell production.
Weeks 31–33: Brain connections multiplying rapidly; hearing fully developed; baby recognizes your voice; practice breathing movements regular.
Weeks 34–36: Lungs maturing significantly; immune system strengthening via placental antibodies; body fat filling out; most babies move to head-down position.
Weeks 37–40: Brain, lung, and liver development continuing right up to and through the due date; vernix and lanugo reducing; baby gaining approximately half a pound per week through week 38.
Physical Symptoms to Expect in the Third Trimester
No two pregnancies are identical, but the following symptoms are common enough in the third trimester that most women will encounter at least several of them.
Lower back pain and pelvic girdle pain affect the majority of pregnant women in the third trimester. Gentle movement, prenatal yoga, swimming, and support belts can help. Severe or debilitating pain is worth discussing with your provider, as physiotherapy referral is appropriate and effective.
Heartburn and indigestion intensify as the uterus continues pressing upward on the stomach. Small frequent meals, avoiding lying down immediately after eating, and elevating the head of the bed can all help. Discuss safe antacid options with your provider.
Swelling (edema) in the feet, ankles, and hands is extremely common and usually benign in the third trimester. Sudden, significant swelling — particularly in the face or hands — or swelling accompanied by headache, visual changes, or upper abdominal pain should be reported to your care provider promptly, as these can be signs of preeclampsia.
Leg cramps — particularly at night — affect many women in the third trimester. Stretching calves before bed, staying hydrated, and ensuring adequate magnesium and calcium intake may help.
Pelvic pressure and lightning crotch — sharp, shooting pains in the pelvis or vaginal area — are caused by the baby's position and the pressure on nerves as the baby descends. They are normal and not harmful, though they can be startling.
Insomnia and disrupted sleep are nearly universal in the third trimester. Side sleeping with a pregnancy pillow, reducing fluid intake in the evenings, and establishing a consistent wind-down routine all help. If sleep deprivation is significantly affecting your functioning or mental health, speak with your provider.
Emotional intensity — anxiety about birth, excitement, impatience, fear — is a normal and largely underacknowledged part of the third trimester. If anxiety is persistent, intrusive, or significantly affecting daily life, please speak with your care provider. Perinatal anxiety is common, treatable, and not something you need to manage alone.
What to Do to Prepare in the Third Trimester
The third trimester is also a season of practical and emotional preparation. Here is what most care providers and experienced mothers recommend doing before your due date arrives.
Take a childbirth education class. Whether in-person or online, a quality birth class — covering labor stages, coping techniques, epidural and medication options, pushing, and the immediate postpartum period — gives you and your support person a shared language and a realistic picture of what lies ahead.
Write your birth plan. A birth plan is not a guarantee — labor rarely follows a script — but the process of writing one helps you clarify your values and preferences and gives your care team useful information about what matters to you. Keep it short (one page), flexible, and focused on preferences rather than demands.
Pack your hospital bag. Aim for 36 weeks. Your bag should cover labor comfort, postpartum recovery for mom, and the baby's coming-home essentials.
Install the car seat. The hospital will not discharge your baby without one. Install it and have it checked by a certified Child Passenger Safety Technician before your due date.
Discuss infant feeding. If you are planning to breastfeed, connecting with a lactation consultant before birth — and knowing how to access one postpartum — significantly improves outcomes. If you are formula feeding, having your supplies ready removes one source of stress.
Prepare your home for recovery. Set up a postpartum recovery station near where you plan to spend the most time — snacks, water, feeding supplies, medications, phone charger, and anything else you will need for the first days and weeks of recovery.
Talk about what you need. With your partner, your family, your care team. The third trimester is the time to have the conversations about help, expectations, and support that will make the postpartum transition more manageable.
Frequently Asked Questions
When exactly does the third trimester start?
The third trimester begins at week 28 of pregnancy, measured from the first day of your last menstrual period. Some sources use week 27 as the starting point, but week 28 is the most widely accepted clinical definition.
How many weeks is the third trimester?
The third trimester spans weeks 28 through 40 — approximately 13 weeks. If pregnancy continues past 40 weeks, which is common, those additional weeks are still considered part of the third trimester.
What are the most common symptoms of the third trimester?
The most common third trimester symptoms include lower back pain, pelvic pressure, Braxton Hicks contractions, heartburn, swelling in the feet and ankles, shortness of breath, frequent urination, insomnia, and fatigue. Emotional intensity — including anxiety, impatience, and anticipatory excitement — is also very common.
When is a baby considered full term?
Full term is defined as 39–40 weeks of pregnancy. Early term is 37–38 weeks. Late preterm is 34–36 weeks. While babies born from 37 weeks onward are generally healthy, research consistently shows that babies who reach 39–40 weeks have better outcomes across multiple measures.
Is it normal to go past your due date?
Yes — very. Only about 5% of babies are born on their due date, and first-time mothers commonly deliver between 40 and 41 weeks. Going past your due date in an uncomplicated pregnancy is not cause for alarm, though your provider will increase monitoring and discuss options as you approach 41–42 weeks.
When do prenatal appointments become more frequent?
Most care providers shift from monthly to every-two-week appointments around week 28–32, then to weekly appointments from approximately week 36 onward. The frequency may increase further if there are any concerns about the pregnancy.
What is lightening and when does it happen?
Lightening (also called engagement or the baby dropping) is when the baby moves lower into the pelvis in preparation for birth. It often happens 2–4 weeks before labor in first-time mothers, and sometimes not until labor begins in women who have given birth before. When it happens, breathing often becomes easier while pelvic pressure and the urge to urinate increase.
What symptoms in the third trimester should I call my doctor about?
Contact your care provider promptly if you experience: sudden or severe swelling particularly in the face or hands; a persistent headache that does not respond to paracetamol; visual disturbances; upper abdominal pain; significant reduction in the baby's movements; signs of preterm labor (regular contractions before 37 weeks); vaginal bleeding; or any symptom that feels sudden, significant, or wrong. Your provider always wants to hear from you when something feels off.
Conclusion: The Third Trimester Is Not the Wait — It Is the Preparation
It is easy to experience the third trimester as something to get through. The discomfort is real. The impatience is real. The exhaustion of waiting for a baby you can already love this fiercely is genuinely hard.
But the third trimester is doing important work — in your baby, who is gaining weight and maturing organs and building the reserves they will need for life outside the womb; and in you, who is preparing — physically, emotionally, practically, and spiritually — for one of the most significant transitions of your entire life.
You do not have to love every week of this. You do not have to feel grateful for the heartburn and the sleepless nights and the pelvic pressure that makes walking uncomfortable. You are allowed to be tired and ready and counting down.
But somewhere in the middle of the countdown, there is also this: you are in the final weeks of carrying this particular child in this particular way. The kicks, the hiccups, the 3 a.m. movements that remind you someone is there — these are also part of it. All of it together is the third trimester.
You are almost there. And everything your body has been doing this whole time has been preparing you for exactly what comes next.
