The third trimester of pregnancy begins at week 28 and continues until you give birth, which typically occurs around week 40. This final stretch of pregnancy brings both excitement and physical challenges as your body prepares for the arrival of your baby.
The Specific Timeline of Pregnancy Trimesters
Pregnancy is divided into three distinct trimesters, each lasting approximately 13-14 weeks. The first trimester spans from week 1 through week 13, the second trimester covers weeks 14 through 27, and the third trimester encompasses weeks 28 through 40 (or until delivery). This final trimester is characterized by rapid fetal growth and significant physical changes for the expectant mother.
Medical professionals calculate pregnancy based on a standard 40-week timeline, starting from the first day of your last menstrual period (LMP). Using this method, the third trimester officially begins on the first day of week 28 and continues until birth. However, it's important to remember that pregnancy duration varies among women, with many delivering between 38 and 42 weeks with perfectly healthy outcomes.
Physical Changes During the Third Trimester
The third trimester brings dramatic physical transformations as your body accommodates your growing baby. Your uterus expands significantly, reaching up to your ribcage and sometimes making breathing more difficult. The additional weight puts pressure on your lower back, hips, and pelvis, potentially causing discomfort or pain. Many women experience Braxton Hicks contractions - practice contractions that help prepare the uterus for labor.
Your baby's movements become more pronounced and frequent during this period. The pattern of these movements becomes an important indicator of fetal health, which is why healthcare providers often recommend kick counting - tracking your baby's movements at specific times of day. A noticeable decrease in movement warrants immediate medical attention.
Weight gain continues in the third trimester, though typically at a slower rate than in the second trimester. Most healthcare providers recommend gaining about 1 pound per week during this final stretch, bringing the total pregnancy weight gain to approximately 25-35 pounds for women who started at a healthy weight.
Fetal Development in the Third Trimester
During the third trimester, your baby undergoes remarkable development. By week 28, your baby weighs about 2-3 pounds and measures approximately 15-16 inches long. The brain continues developing rapidly, and the lungs prepare for breathing air. Your baby's eyes can open and close, and they can respond to light.
By week 32, your baby has developed fully formed fingernails and toenails. The bones have hardened, though the skull remains soft and flexible to facilitate passage through the birth canal. The digestive system matures, and your baby begins to store vital minerals like iron and calcium.
Around week 36, most babies move into the head-down position in preparation for birth. The protective vernix coating and lanugo hair that covered your baby's skin begin to disappear. By week 40, your baby typically weighs between 6-9 pounds and measures approximately 19-21 inches in length, though these measurements vary considerably among healthy newborns.
Important Medical Check-ups and Monitoring
Healthcare providers increase the frequency of prenatal visits during the third trimester. While appointments might have been scheduled every four weeks during earlier trimesters, they typically occur every two weeks from weeks 28-36 and then weekly from week 36 until delivery.
These appointments include standard measurements of your blood pressure, weight, and fundal height (the distance from your pubic bone to the top of your uterus). Your provider will check your baby's heart rate and position through physical examination or ultrasound. They will also test your urine for protein and glucose to monitor for conditions like preeclampsia and gestational diabetes.
Around week 36, your provider may perform a Group B Streptococcus (GBS) test, which involves a painless swab of your vaginal and rectal areas. This test screens for bacteria that rarely causes problems in adults but can be serious for newborns if transmitted during birth.
Preparing for Labor and Delivery
As you progress through the third trimester, your body naturally prepares for labor. The cervix begins to soften (efface) and open (dilate) in preparation for birth. Many women experience lightening or dropping - when the baby descends lower into the pelvis, which might make breathing easier but increase pressure on the bladder.
Creating a birth plan during this trimester helps communicate your preferences to your healthcare team. This document outlines your wishes regarding pain management, labor positions, immediate newborn care, and other aspects of the birth experience. While flexibility is important since births rarely go exactly as planned, having a birth plan promotes informed decision-making.
The third trimester is also the optimal time to pack your hospital bag, install your car seat, and prepare your home for the newborn's arrival. Many expectant parents use this time to finalize nursery preparations, stock up on essential supplies, and arrange for help during the first weeks postpartum.
Managing Common Third Trimester Discomforts
The final trimester brings unique physical challenges. Sleep disturbances become common as finding comfortable positions grows increasingly difficult. Many healthcare providers recommend sleeping on your left side with pillows supporting your back and between your knees to improve circulation and comfort.
Heartburn and indigestion often intensify as your growing uterus puts pressure on your stomach. Eating smaller, more frequent meals and avoiding spicy or acidic foods can help manage these symptoms. Some women experience intensified back pain and round ligament pain as the body's center of gravity shifts and ligaments stretch to accommodate the growing baby.
Swelling (edema) in the feet, ankles, and hands is normal during the third trimester due to increased blood volume and pressure from the uterus on major blood vessels. Elevating your feet, staying hydrated, and wearing compression stockings can provide relief. However, sudden or extreme swelling, especially if accompanied by headaches or vision changes, requires immediate medical attention as it could indicate preeclampsia.
When to Call Your Healthcare Provider
While many discomforts in the third trimester are normal, certain symptoms warrant immediate medical attention. Contact your healthcare provider if you experience:
Vaginal bleeding or leaking fluid, which could indicate placental problems or rupture of membranes
Severe or persistent headaches, vision changes, or sudden swelling, which might signal preeclampsia
Decreased fetal movement over a two-hour period
Regular, painful contractions before 37 weeks, which could be preterm labor
Severe abdominal pain or vomiting
Prompt attention to these warning signs can prevent complications and ensure the best outcomes for both mother and baby.
Conclusion
The third trimester marks the final stretch of pregnancy, beginning at week 28 and continuing until birth. This period brings significant physical changes and preparations as your body and baby ready themselves for delivery. While the discomforts of the third trimester can be challenging, they are temporary and signal that you're getting closer to meeting your baby. Regular prenatal care, attention to warning signs, and appropriate preparations can help ensure a healthy birth experience.
FAQs
Can the third trimester start earlier than 28 weeks?
Medical professionals define the third trimester as beginning at week 28, but some women may experience third-trimester symptoms earlier. The division into trimesters is somewhat arbitrary and designed to track developmental milestones rather than strict physical changes.
What should I prioritize preparing during the third trimester?
Focus on essential preparations like finalizing your birth plan, packing your hospital bag, installing your car seat, and setting up a safe sleeping space for your newborn. Arranging postpartum support and meal preparation can also significantly reduce stress after birth.
How can I distinguish between Braxton Hicks contractions and real labor?
Braxton Hicks contractions typically feel like a tightening of the abdomen that comes and goes irregularly. They usually don't increase in intensity or frequency and often subside with position changes or hydration. Real labor contractions gradually become stronger, longer, and closer together, and typically continue regardless of activity changes.
