At What Week Does The Third Trimester Begin? | Week 28

Why This Week Matters

Week 28 marks the shift into the final stretch, when growth speeds up and appointments come more often. Care plans and birth prep move center stage.

At What Week Does The Third Trimester Begin? Facts

Most obstetric groups define the third trimester as week 28 through birth. A few hospital sites and parenting portals still use week 27. That gap usually comes from rounding “months” into weeks or counting methods that start from conception rather than the last menstrual period. In routine care, providers schedule third-trimester testing and checkups from 28 weeks onward, so using 28 keeps you aligned with common practice. When in doubt, follow your own team’s plan and the timing on your chart or app.

Trimester Timing At A Glance

Use the chart below to place your weeks. It reflects the common medical convention that sets the third trimester at 28 weeks and zero days.

Table #1: within first 30% (≥7 rows, ≤3 columns)

Stage	Weeks (LMP)	What This Window Covers
First Trimester	0–13+6	Implantation, early organ formation, early screening
Second Trimester	14–27+6	Anatomy scan, movement starts, glucose screening window
Third Trimester	28–40+6	Rapid growth, birth planning, late-pregnancy monitoring
Early Term	37–38+6	Not yet full-term; higher breathing/feeding issues than 39+
Full Term	39–40+6	Preferred window for planned birth when no medical reason
Late Term	41–41+6	Closer watch; many discuss induction options
Post-Term	42+0 and beyond	Rare; delivery usually recommended

What Changes Right After Week 28?

From week 28, growth, movement, and monitoring pick up pace. You’ll likely notice stronger kicks, more rib pressure, and more frequent bathroom trips. Clinics start glucose reviews earlier, but anemia checks, Rh screening, and vaccine statuses are often revisited around this point. Visit cadence also tightens as you approach week 36.

Why Some Sites Say Week 27

Two counting quirks explain the confusion:

• Month math: Some sites convert “7 months” into 27 weeks. That’s a rough conversion and not how medical dating works.
• Different start point: A few pages count pregnancy from conception, which lags the clinical clock by about two weeks. On that scale, “26 after conception” roughly matches “28 by last period.”

Parents often ask, “at what week does the third trimester begin?” during the glucose visit; the short, practical answer used in clinics is week 28.

Third Trimester Start Week — Clear Rules And Useful Nuance

Here’s a simple way to think about it. If your prenatal record shows 28+0, you’re in the third trimester. If it shows 27+6, you’re still in the second. Crossing midnight into 28+0 flips the label. That’s how most guideline pages describe it (ACOG trimester timing), and it’s how appointment schedules are built.

For a practical weekly view that also treats 28 as the start, see the NHS week-28 overview (NHS week-28 guide).

Third Trimester Body Changes

• Breathing feels harder as the uterus lifts the diaphragm.
• Sleep gets choppy; side-lying with a pillow between knees can help.
• Swelling shows up in feet and hands, especially late in the day.
• Braxton Hicks tightenings become more noticeable.
• Heartburn and reflux tend to flare after larger meals.

None of these tips replace care. Report new, severe, or sudden symptoms right away.

Fetal Milestones From 28 Weeks

• Rapid weight gain and fat layering
• Stronger, regular movement patterns
• Maturing lungs and brain
• Head-down position becomes more common through weeks 32–36

Report any movement drop the same day.

Appointments And Tests In Late Pregnancy

Typical schedules go every two to four weeks until week 36, then weekly until delivery. Your exact plan depends on your health, prior outcomes, multiples, and any conditions that need extra eyes. Screening for anemia, gestational diabetes follow-ups, group B strep swabbing near week 36–37, and discussions about birth preferences all land in this window.

What Counts As “Week 28” If My Dates Changed?

Ultrasound can adjust your estimated due date early on. After that, the due date stays fixed while weekly labels tick forward. If your provider updated your date, your “28” moves with it. The third trimester starts when your chart says 28+0 based on the current due date.

Month-By-Month Versus Week-By-Week

Rounding months to weeks creates mismatches. Months vary in days; weeks are always seven days. Medical records use weeks and days to prevent drift. That’s why “month seven” on a blog might not match your visit schedule or test dates.

Safety Checks To Step Up At 28 Weeks

• Blood pressure: learn your clinic’s warning thresholds.
• Swelling plus headache or vision changes: urgent call.
• Kick counts: learn your baby’s usual rhythm.
• Signs of preterm labor: pelvic pressure, low back ache, cramps, or fluid leaking.
• Vaccines: ask about the timing your clinic follows.

Quick calls save worry. Never wait if you feel something is off.

Sleep, Nutrition, And Movement

Short naps help. Smaller, more frequent meals tame reflux. Aim for iron-rich foods with vitamin C for absorption, and get enough protein. Gentle movement—walking, prenatal yoga, swimming—supports back comfort and circulation. If anything causes pain or dizziness, stop and call your provider.

Birth Planning Starts To Get Real

Talk through your hospital or birth center’s intake steps, visitor rules, and newborn care routine. Share preferences with your partner or support person and keep a copy in your bag. Confirm your route and a driver. If you’re considering epidural, nitrous, or unmedicated methods, jot your thoughts and revisit at each visit.

When The Label Matters For Benefits And Paperwork

Some leave policies, insurance forms, and travel protections use trimester language. Using week 28 as the start keeps you aligned with many employer and insurer templates. If a form defines trimesters differently, follow that document’s wording and attach a note from your clinician if needed.

Third Trimester Start In Special Situations

• Twins or more: visits are often more frequent; some tests move earlier.
• Conditions like hypertension or diabetes: monitoring tightens and birth timing may be discussed ahead of due date.
• Prior preterm birth: you may see extra checks for cervical length or early signs of labor.

Third Trimester Red-Flag Symptoms

Call right away for any of the following:

• Reduced movement from your usual pattern
• Vaginal bleeding or fluid leakage
• Severe headache, vision changes, or right-upper-belly pain
• Painful, regular contractions before 37 weeks
• Fever, chest pain, or trouble breathing

A nurse line can triage, but emergency care is appropriate when symptoms are severe.

Hospital Bag And Home Prep Timeline

• Weeks 28–30: start a packing list, choose a pediatrician, and check car seat fit.
• Weeks 31–33: wash newborn clothes, stock pads, set up a bassinet.
• Weeks 34–36: pack the bag, install the car seat, and prep freezer meals.
• Weeks 37–40: keep fuel in the car, charge phones, and review your birth plan.

Common Myths About The Third Trimester Start

• “My app says 27 weeks.” Apps vary; check their settings. Some use different day-count rules or start from conception.
• “Months are clearer than weeks.” Weeks prevent drift. Medical tests and visits map to weeks and days, not calendar months.
• “Movement slows late in pregnancy.” Movements may feel different as space tightens, but frequency shouldn’t fall.

Sample Questions To Ask At Your 28-Week Visit

• How often should I feel movement, and what’s my next step if it changes?
• What is my visit schedule from now to delivery?
• Do I need iron, aspirin, or other medicines based on my labs?
• When will we do group B strep testing?
• What are your after-hours numbers and the fastest way to reach the team?

Prenatal Visit Cadence And Tests After 28 Weeks

Most babies arrive between weeks 37 and 42. “Due date” marks 40+0 by convention, not a promise. Many planned deliveries aim for 39 weeks or later unless there’s a medical reason to act sooner; see ACOG’s definition of term pregnancy for the week ranges many hospitals use.

Table #2: after 60% (≤3 columns)

Gestational Week	Typical Care	Notes
28–31	Visit every 2–4 weeks; anemia check; review vaccines	Flu and whooping cough shots depend on season and guidance
32–33	Growth and blood pressure check; movement review	Ask about kick counts and when to call
34–35	Position check; birth preferences; lab review	Pack your bag and confirm a pediatrician
36	Weekly visits often begin; group B strep swab	Ask how results change labor care
37	Weekly visit; cervical check only if needed	Talk about signs of labor and when to come in
38	Weekly visit; fetal position and fluid questions	Discuss membrane sweep only if appropriate
39	Weekly visit; induction choices if medical reasons	Many planned births target 39–40+6
40–41	Testing for some; induction planning	Timing depends on your health and local protocol

Due Date Windows And Birth Timing

Most babies arrive between weeks 37 and 42. “Due date” marks 40+0 by convention, not a promise. Many planned deliveries aim for 39 weeks or later unless there’s a medical reason to act sooner.

When Plans Change

If new symptoms, lab results, or ultrasound findings arise, your team may shift visit timing, add testing, or discuss earlier delivery. That isn’t a failure of planning; it’s how good care adapts. Ask for plain-language summaries of each change.

The Bottom Line

Label the third trimester at week 28. If you’re asking, “at what week does the third trimester begin?”, mark 28+0 on your chart. Use weeks and days for decisions, ask about anything that feels different, and keep your bag list handy. Small steps lower stress and help you enjoy the final stretch. Bring questions to visits