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When Does the Third Trimester Start?

When Does the Third Trimester Start?

When does the third trimester start?

As you get to the last trimester of your pregnancy, there are things you are used to by now and yet still many new surprises. Your baby continues to grow and develop through these vital weeks. You continue to grow and develop as you wait for your due date to approach.   The third trimester weeks are weeks 28-40 of your pregnancy. Some women will go into labor before that, and 38 weeks is considered full term per the American Pregnancy Association.  

The first trimester is marked by morning sickness and surprise. The second trimester can mean more energy and prep. The third trimester includes birth prep, noticeable baby movement, nesting, and new aches and pains.  

When is the Third Trimester?

The average length of pregnancy is 40 weeks. The dates are calculated by adding 40 weeks to the date of your last menstrual period. The third trimester is 13 weeks long. If your baby decides to arrive early, your third trimester will be longer. If your baby stays in the womb longer than expected, your third trimester may be up to 15 weeks long.  

Q: When does the third trimester start?
A: The third trimester starts in week 28 of pregnancy.  

Q: When does the third trimester end?  
A: The first trimester ends at birth! Full term is anytime after 39 weeks.  

Q: What is the length of the third trimester?  
A: The first trimester is 13 weeks long if you birth at 40 weeks gestation

Baby Changes in the 3rd Trimester

In the last three months of your pregnancy, your baby is continuing to develop vital organs, bone strength, and senses. By the end of the third trimester, your baby will be prepared for delivery. 

Q: When is a baby considered full-term?
A: You baby is considered full term at 38 weeks per the American Pregnancy Association. 

Q: When are baby’s bones fully developed?
A: Your baby’s bones continue to store calcium and strengthen bones in the third trimester. However, the skull does remain softer in order to accommodate for birthing in the vaginal canal. 

Q: Can my baby hear me?
A: Babies can heard sound in the second trimester. However, responsiveness increases in the third trimester and babies pick up on even more sensory experiences.  

Q: When is a baby considered premature?
A: The World Health Organization considers any birth before 37 weeks as premature/preterm. 

Q: When can a baby breathe on his/her own?
A: The lungs continue to develop throughout the third trimester. This is why some preterm babies may struggle with respiratory issues that need attention. However, some babies born after 36 weeks can breathe independently without concern. 

The Seventh Month of Pregnancy

The seventh month of pregnancy is marked by sensory reactions and fat increases. Your baby will become more responsive to light and sound. He can even taste what you taste. You will likely feel your baby’s movement and maybe even notice reactions to your favorite music. This is a great time for story reading and song singing to your baby. Your baby is also opening and closing eyelids and has more body temperature control.  

The Eighth Month of Pregnancy

During the eight month of pregnancy, your baby is developing in the brain and the bones. As the bones are fully formed, you baby is preparing for life outside of the womb. During this time the lanugo, the small hair covering your baby’s body, starts to fall off. Your baby is also enjoying a day in the amniotic sac spa- completing the formation of fingernails and toenails.  

The Ninth Month of Pregnancy

During the last month, your baby is getting positioned for delivery. The lungs complete formation during this time and your baby is taking up most of the space inside the amniotic sac. Your baby is also preparing the first poop, called meconium. As your baby decends further into the pelvis, you may experience lightning crotch and frequent urination as discussed below.  

Changes in Mom During the 3rd Trimester Weeks

It is no surprise that mom continues to experience changes throughout the 3rd trimester. As an expecting mom you might experience the following symptoms:  

-Braxton hicks
-Frequent urination
-Shortness of breath
-Joint pain
-Swelling of the ankles, fingers, face 

Some women still feel tons of energy in the third trimester and others are ready for the pregnancy to be over. Just know that if you are feeling achy and swollen, it will not last forever. 

As you get closer to delivery, you may experience lightning crotch, which is at term used to describe the feeling of the baby moving down into the birth canal. This can mean you can breathe more easily, but may have more pain and urination.   

Q: How much weight should I gain in the third trimester?
A:  As you near birth, your weight gain will increase in the third trimester. This is because your baby is growing rapidly. Also your body prepares for birth with a growing uterus, a growing placenta, increased blood levels, and more amniotic fluid. 

Q: Why are my breast leaking before the baby is born?
A: Some women have leaking breasts prior to giving birth. This is colostrum leaking as your body starts producing milk for breastfeeding.  

Q: How do I know if contractions are Braxton-hicks?
A:  Talk with your provider about distinguishing between Braxton-hicks and labor contractions. You will likely be able to tell the difference based on regularity and frequency.  

Q: When should I start kick counts?
A: Many providers recommend kick counts all throughout the third trimester.

Staying Healthy in the Third Trimester

Throughout all of your pregnancy, you want to ensure that you are making healthy choices for yourself and your baby. During this time American College of Obstetricians and Gynecologists recommends that mothers stop any smoking, alcohol consumption, drug usage, and avoid X-Rays.You should also continue a healthy diet that is rich in fruits, vegetables, protein and fiber. Throughout your entire pregnancy, avoid high mercury fish, unpasteurized dairy, and high levels of caffeine, cat litter, and uncooked meat.  Make sure you are eating an additional 300 calories per day than your non-pregnancy normal to accommodate for the extra needs.  

When to Call Your Doctor

It can be alarming to go through so many changes and feel your body through late pregnancy. As you approach birth, you likely have questions about what contractions are labor and what are not.  

Talk with your doctor about the following complications that can impact the third trimester. Know what signs to look out for and if you have an “off” feeling, trust your gut and check in with your provider.  

Complications in the 3rd Trimester can include:  

-Intrauterine Growth Restriction
-Gestational Diabetes
-Preterm Labor
-Placenta Previa/ Placenta Abruption
-Premature Rupture of the Membrane (PROM)  

During these last weeks of your pregnancy, enjoy feeling and seeing your baby and body grow. If you find yourself awake at night nesting, make sure you check out all the new baby goods at Zulily. As you prepare for your baby’s arrival and this next stage of your life, take time to rest and reflect. You’ve got this momma! 


About The Author

Chelsea Skaggs

Chelsea Skaggs is a postpartum advocate and coach who is committed to helping women kick the pressure to be "Pinterest Perfect" and have real, raw conversations to acknowledge and empower the postpartum experience. She provides small group coaching, eCourses, online communities and helps other women start motherhood-centered businesses. She believes that normalizing and empowering all the changes in life after baby can change the world and leads that effort at

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