Third Trimester: Symptoms and Baby Development
You’ve reached the final stretch (literally!). This is the most exciting and suspenseful trimester of pregnancy. Read on to learn all about what happens in the third trimester of pregnancy, including fetal development and common symptoms you may experience. Pregnancy is a period that begins from the first day of the last menstrual cycle and ends with the birth of the baby, which is also termed intrauterine life.
When Does the Third Trimester Start and How Long Is It?
Officially, the third trimester runs from 28 to 40 weeks of pregnancy, lasting about 13 weeks, but in reality, the third trimester ends when your baby is born. Although there isn’t an exact answer, as it depends on when your baby actually arrives, the third trimester is considered about three months long. If you’re wondering what months the third trimester includes, it’s generally months 7, 8, and 9 of your pregnancy.
When you reach the start of 39 weeks, your pregnancy is considered full-term. If a baby is born before 37 weeks, this is called a preterm birth. As you enter the third trimester, it’s important to know the signs of preterm labor, just in case your little one decides to make an early appearance.
Your Baby’s Development in the Third Trimester
In the third trimester of pregnancy, your little one continues to grow at a fast pace. In fact, your baby will gain about half of their birth weight during the final months of your pregnancy. By the time your baby is born, they may weigh between 5 1/2 and 9 1/2 pounds.
Fetal Development Milestones
- 28 Weeks: When you’re around 28 weeks pregnant, your little one can open and close their eyes, and they may have eyelashes.
- 30 Weeks: At this point, they may start to shed the coat of fine hair called lanugo. Most babies are born with a bit of vernix, a protective waxy coating, still covering their skin. Around this week of pregnancy, your baby may also start to grow normal hair on their head.
- 31 Weeks: Your baby’s brain is maturing and growing rapidly this week. Their brain can now control body temperature, so they no longer have to rely on the temperature of the amniotic fluid.
- 34 Weeks: Around the time you’re 34 weeks pregnant, or soon after, your little one will most likely turn head-down in preparation for birth.
- 39 Weeks: By the time you reach the start of 39 weeks, your baby is considered full-term and they’re ready for the outside world now.
By 36 weeks, they’ll have done such a good job of growing that they won’t have much room to move throughout the rest of the pregnancy!
Pregnancy and Birth over 35
There are benefits to having a baby after 35 years old. Older women and birthing people are more likely to show an increase in happiness after having a baby than those younger than them (Myrskylä et al, 2017). However, as people get older, the chance of other health issues rises. Conditions associated with higher maternal age include:
- Chronic hypertension
- Gestational diabetes
- Placenta praevia
The chance of having a baby with a fetal anomaly such as Down, Edwards or Patau’s syndromes increases from the age of 25. It is highest over the age of 45. Older women and birthing people are also more likely to have multiple pregnancies (such as twins or triplets).
Risks and Clinical Care
The chance of miscarriage increases to older mothers and fathers, and is most likely if the mother is over 35 and the father is over 40. Pre-eclampsia and gestational hypertension are moderate risk factors for women and birthing people over 40. Doctors may offer induction earlier (at 40 weeks rather than 41-42 weeks) to reduce any possible impact on the baby.
Comparison of Birth Categorization
| Category | Week Range |
|---|---|
| Preterm birth | Before 37 weeks |
| Full-term pregnancy | Starts at 39 weeks |
| Post-term birth | Up to 42 weeks |
Fertility and Age Factors
For those over 35 using artificial insemination, between 33 and 50 in 100 are pregnant after one cycle (or one year of IUI), and between 50 and 67 in 100 are not. After the age of 35, the age of the father is as much a factor as the woman’s age. But you are still more likely to have a healthy pregnancy than a miscarriage even if you are over 40.