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Third Trimester Guide — Pregnancy Weeks 28 to 40
Navigate your third trimester (weeks 28-40). Learn about baby positioning, Braxton Hicks contractions, and signs of labor.
Interactive Calculator
Your Pregnancy Details
First day of your last period. Gestational age is calculated from this date.
Your Pregnancy Timeline
Trimester
Weeks 1–13
~38 weeks
Days Remaining
266days
Week 15% completeWeek 40
0
4
8
12
16
20
24
28
32
36
40
Weeks 1–13
Major organs and systems form. Common symptoms include fatigue, nausea, and breast tenderness.
Tri 2
Second Trimester
Weeks 14–27
Baby grows rapidly. Movement felt. Symptoms often improve. Anatomy scan at 18–22 weeks.
Tri 3
Third Trimester
Weeks 28–40+
Baby continues maturing. Braxton Hicks, back pain, and nesting instinct are common.
Pregnancy Milestones
✓
Conception
Week 0
✓
Embryo Implants
Week 2
6
Heartbeat Detectable
Week 6
12
NT Scan
Week 12
20
Anomaly Scan
Week 20
24
Viability Threshold
Week 24
28
Third Trimester Begins
Week 28
37
Full Term
Week 37
40
Due Date
Week 40
You are in the First Trimester (Weeks 1–13). You are week 3, day 0 of your pregnancy. Approximately 266 days (38 weeks) remain until your due date.
The formula
The formula
What Gestational Age Measures
Pregnancy is measured in gestational age, which counts from the first day of your last menstrual period (LMP), not from the date of conception. This is the standard method used by obstetricians and midwives worldwide because the LMP date is typically the most reliably remembered reference point. A full-term pregnancy is defined as 40 weeks (280 days) from LMP, though only about 5% of babies are born on their exact due date.
Gestational age is the universal reference for scheduling prenatal tests, assessing fetal development milestones, determining viability, and planning delivery. Understanding your current pregnancy week helps you know what to expect at each stage of your pregnancy journey and ensures timely prenatal care.
The Gestational Age Formulas
Days Since LMP = Today − LMP
Current Week = Floor(Days Since LMP / 7) + 1
Current Day = Days Since LMP mod 7
Days Until Due Date = 280 − Days Since LMP
LMP = first day of last menstrual periodFloor = round down to nearest integer
Formula Source
This calculator uses the **standard gestational age calculation (days from LMP to today converted to weeks)** from **ACOG (American College of Obstetricians and Gynecologists)** guidelines.
Reference URL: https://www.acog.org/womens-health/infographics/pregnancy-timeline
Last Verified: 2026-07-30
Worked Example
If your LMP was January 1, 2026 and today is August 9, 2026: Days since LMP = 220 days. Current Week = Floor(220 / 7) + 1 = Floor(31.4) + 1 = 31 + 1 = 32. Current Day = 220 mod 7 = 3. This means you are 32 weeks and 3 days pregnant. Days until due date = 280 − 220 = 60 days remaining (approximately 8 weeks and 4 days). Your due date calculated from Naegele's Rule (LMP + 280 days) would be October 8, 2026.
Trimester Breakdown
| Trimester | Weeks | Key Developments |
|---|---|---|
| First Trimester | Weeks 1–13 | Organ formation, heartbeat detectable, neural tube closure |
| Second Trimester | Weeks 14–27 | Growth, fetal movement felt, anatomy scan, viability threshold |
| Third Trimester | Weeks 28–40+ | Rapid weight gain, lung maturation, head-down positioning, labor preparation |
FAQ-Style Explanations
What is the difference between gestational age and fetal age? Gestational age is measured from LMP (about 40 weeks total) and is the standard medical reference. Fetal age (or conceptual age) is measured from conception and is about 2 weeks less than gestational age. This calculator uses gestational age, which is what your healthcare provider will use.
What if I have irregular cycles? For people with regular 28-day cycles, LMP-based calculation is reasonably accurate. However, if you have irregular cycles or are unsure of your LMP, first-trimester ultrasound dating is more reliable. Crown-rump length (CRL) measurement between 7–13 weeks is considered the most accurate method for dating a pregnancy, with an error margin of only ±3–5 days.
Is it normal for my calculated weeks to differ from my ultrasound? Yes, it is common for LMP-based calculations to differ from ultrasound dating. If the difference is more than 5–7 days in the first trimester, most providers will use the ultrasound date as the reference. Discrepancies often arise from irregular ovulation timing or inaccurate LMP recall.
Known Limitations
- Assumes a regular 28-day menstrual cycle with ovulation on day 14. Cycles shorter or longer than 28 days will shift the actual ovulation date and therefore the true gestational age.
- Relies on accurate recall of the LMP date, which may not be reliable for all individuals.
- Does not account for variations in the luteal phase length, which can range from 10–16 days across different individuals.
- For IVF pregnancies, the gestational age should be calculated from the transfer date and embryo age, not from LMP.
- The 40-week (280-day) standard is an average — normal pregnancies range from 37 to 42 weeks. Post-term pregnancies beyond 42 weeks may require medical intervention.
- First-trimester ultrasound dating is considered more accurate than LMP-based calculation and is preferred when available, especially for clinical decision-making.
Scenario guide
Scenario guide
Weeks 28 to 32 — Rapid Growth and Lung Maturation
The third trimester begins at week 28 and is marked by the fastest rate of fetal weight gain in the entire pregnancy. Between weeks 28 and 32, the baby gains approximately 200 to 250 grams per week and the lungs accelerate surfactant production, the protein-lipid substance critical for breathing after birth. By week 28, the fetus weighs about 1 kg (2.2 lb) and measures about 35.6 cm (14 inches) crown to heel. The eyes can open and close, the circadian rhythm begins developing, and the baby can respond to sounds with movement or changes in heart rate. ACOG recommends that kick count monitoring begin at week 28, with each pregnant individual tracking fetal movement daily and reporting any significant decrease in activity to their provider. The GBS (group B streptococcus) screening swab is typically performed between weeks 35 and 37.
Weeks 33 to 36 — Engagement and Braxton Hicks
Between weeks 33 and 36, the fetus typically begins engaging — meaning the head descends into the pelvis in preparation for birth. For first-time mothers, this engagement, often called "lightening," can occur 2 to 4 weeks before labor begins. In subsequent pregnancies, engagement may not occur until labor starts. Braxton Hicks contractions — irregular, painless tightening of the uterine muscle — become more frequent and noticeable during this window. They differ from true labor contractions in that they do not increase in frequency, intensity, or duration over time, and they typically ease with position changes or hydration. By week 36, the fetus weighs approximately 2.6 kg (5.7 lb) and is about 45.6 cm (18 inches) long. At this point, prenatal visits shift from every 2 weeks to weekly, continuing through delivery.
Weeks 37 to 40 — Full-Term and Delivery Preparation
Week 37 marks the beginning of early-term pregnancy, and by week 39 to 40 the baby is full-term — the optimal window for delivery. Between weeks 37 and 40, the baby typically drops another 2 to 3 cm into the pelvis, the cervix begins to soften and thin (efface) in preparation for labor, and hormones like prostaglandins and relaxin prepare the body for birth. The fetus shifts fat deposits around the heart and gut to help regulate body temperature after birth. By week 40, the average baby weighs about 3.4 kg (7.5 lb) and measures about 50 cm (20 inches) long. ACOG recommends avoiding elective delivery before 39 weeks unless medically indicated, as the risk of respiratory complications, feeding difficulties, and hypoglycemia is higher for babies born before 39 weeks.
Late-Term and Post-Term Definitions
Understanding late-term and post-term classifications helps expectant parents and providers make informed decisions about delivery timing. ACOG defines late-term pregnancy as weeks 41 0 days through 41 6 days, and post-term as 42 weeks 0 days or beyond. After week 41, the placenta's efficiency declines, amniotic fluid volume may decrease (oligohydramnios), and the risks of macrosomia, meconium aspiration, and stillbirth rise. Most obstetric guidelines recommend active monitoring with non-stress tests and amniotic fluid assessments between weeks 41 and 42, and induction of labor is strongly recommended if the pregnancy reaches 42 weeks. The risk of stillbirth increases significantly after 42 weeks, reaching approximately 1 per 100 pregnancies compared to about 0.3 per 100 at 40 weeks.
Signs of Labor
Recognizing the signs of true labor is one of the most important skills to develop in the third trimester. The 5-1-1 rule is a common guideline for when to go to the hospital: contractions occurring every 5 minutes, lasting about 1 minute each, for at least 1 hour. Other signs include rupture of membranes (water breaking), which may present as a sudden gush or a steady trickle of fluid; bloody show, which is the passage of a mucus plug tinged with blood as the cervix begins to dilate; and a strong, persistent urge to push. ACOG notes that water breaking without contractions should prompt a call to the provider within 24 hours, as the risk of infection increases once the amniotic sac is ruptured. False labor (Braxton Hicks) typically does not progress, eases with rest or hydration, and is felt primarily in the front of the abdomen, whereas true labor contractions increase in intensity, are often felt in the back and radiate to the front, and do not resolve with activity changes.
FAQ
Frequently Asked Questions
The 5-1-1 rule is a common guideline for timing when to go to the hospital during active labor. It means contractions are occurring every 5 minutes apart, each lasting about 1 minute, and this pattern has been consistent for at least 1 hour. This typically corresponds to the active phase of labor when cervical dilation is progressing more rapidly. However, this is a general guideline and your provider may give you different instructions based on your personal circumstances. Always call immediately if your water breaks, if you notice vaginal bleeding heavier than light spotting, if you experience decreased fetal movement, or if you have any other concerning symptoms.
Braxton Hicks contractions are irregular, unpredictable tightening of the uterine muscle that typically begins in the second or third trimester. They are usually painless or mildly uncomfortable, do not increase in frequency or intensity over time, and often ease with position changes, hydration, or rest. True labor contractions, by contrast, are regular, progressively increase in frequency, duration, and intensity, do not ease with rest or position changes, and are often felt in the lower back and radiate to the front. Braxton Hicks contractions are felt primarily in the front of the abdomen, while true labor contractions tend to start in the back.
When the baby drops or engages, the head descends into the pelvis and settles into position for birth. For first-time mothers, this "lightening" typically occurs 2 to 4 weeks before labor begins, while for mothers who have given birth before, engagement often does not occur until labor starts. Lightening can bring relief from shortness of breath and heartburn as the uterus shifts downward, but it may increase pressure on the bladder and legs. Your provider may note engagement during prenatal exams and assign a presentation (cephalic, breech, or transverse) that helps inform delivery planning.
Group B streptococcus (GBS) is a bacteria that naturally colonizes the gastrointestinal and genital tracts of about 25% of pregnant individuals. The GBS screening involves a swab of the vagina and rectum, typically performed between weeks 35 and 37 of pregnancy. If the test is positive, intravenous antibiotics are administered during labor to reduce the risk of vertical transmission to the baby, which can cause serious infections including sepsis, pneumonia, and meningitis. With appropriate intrapartum antibiotic prophylaxis, the rate of early-onset GBS disease in newborns has decreased by more than 80% since the guidelines were implemented in 1996.
A pregnancy is considered post-term at 42 weeks 0 days or beyond. After 41 weeks, placental efficiency declines and the risk of oligohydramnios (low amniotic fluid), macrosomia (large baby), and meconium aspiration increases. The risk of stillbirth rises from approximately 0.3 per 100 pregnancies at 40 weeks to about 1 per 100 at 42 weeks. Most obstetric guidelines recommend active monitoring with non-stress tests and amniotic fluid index measurements between weeks 41 and 42, and induction of labor is strongly recommended at or before 42 weeks. ACOG classifies pregnancies at 41 weeks 0 days to 41 weeks 6 days as "late-term" and recommends active management during this period.
Keep going
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