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Your Baby’s Development by Trimester

Jan 14, 2022 | 4 Minute Read

Your womb (uterus) is baby’s first “home” for approximately 40 weeks. Pregnancy typically lasts 280 days, or 40 weeks, beginning with the first day after your last menstrual period. Your first peek into baby’s first “home” will be via ultrasound, which is usually done between weeks 8-12, especially if you don’t know the date of when your last menstrual period started.

Your 1st Trimester: Weeks 1-12

Wow, while you may be dealing with fatigue and nausea, a lot is going on inside your uterus. During this first trimester period, your baby will grow from an embryo 1/5th the size of the period at the end of this sentence to a nice juicy plum, as some pregnancy books like to characterize baby’s growth by comparing your little peach to apples and oranges.

Fertilization of an egg (or ovum) begins when sperm unites with your egg, and voila! conception. Once fertilization occurs, baby goes through rapid growth—from a zygote (first couple of weeks) to an embryo (end of the second week through the 8th week) to a fetus (end of the 8th week until birth).

The unique characteristics that your baby will be born with, and their biological sex, are passed on during conception. A male’s sperm carries both XX and XY chromosomes, so an XX zygote will biologically become a female, and an XY zygote will biologically become a male.

By days 7-8, the embryo implants into the upper part of your uterus. During the next few weeks, the embryo will undergo dramatic changes all occurring well before you even suspect you may be pregnant. Around days 8-10 after conception, a pregnancy test can detect the pregnancy hormone, human chorionic gonadotropin, or hCG, and share the good news of your baby’s creation.

During weeks 3-4, baby’s external features including their eyes, ears and nose develop, and their nervous system, digestive tract and circulatory system develops. If all is going well, typically a heartbeat can be detected by the 6th week and the heart is completely developed by week 8, as well as all organ systems.

It’s during these first highly sensitive 8 weeks, when all organs are being developed, that your baby needs protection from external environmental factors, such as drugs, alcohol, smoking, infections, stress, and other exposures that can cause birth disorders. You may not even know you’re pregnant yet at week 8, so if you’re planning on getting pregnant, seek preconception counseling with your healthcare provider to ensure you enter pregnancy as healthy as possible so that all the rapid development that occurs in the first trimester can be as healthy best as possible.

Week 9 begins the “fetal” period that lasts until baby is born. By this stage, all the organs and structures are forming and will continue to develop and grow. Your fetus is nourished by the placenta (which is developed by week 12 and provides a direct connection from the mother to baby through her umbilical cord). The placenta continues to grow and develop through week 20.

The placenta produces hormones to maintain the pregnancy and, eventually, start labor. It supplies oxygen and nutrition to baby and removes waste products produced by baby. Your baby is suspended in your uterus in amniotic fluid (the “bag of waters”). This fluid is essential for baby’s internal life in the womb: It cushions baby, helps maintain its temperature, promotes fetal movement, and is essential for fetal growth and development.

The placenta passes almost all the things that you eat, drink or inhale to your baby, so be careful and consult with your midwife or physician on medications, foods and drinks you may be uncertain of regarding their safety for baby.

Your baby is also preparing its dazzling smile you won’t begin to see for quite some time during weeks 9 through 12 as teeth buds begin to form. Fingers and toes are growing, baby begins to excrete urine and to move, kick and swallow. Baby’s busy in there!

Your 2nd Trimester: Weeks 13-26

Ah, middle pregnancy—hopefully by this time any nausea you may have experienced has gone and you’re getting more comfortable with your growing bump. During these weeks, your baby grows from the size of a peach to an eggplant.

Baby’s bones become harder, and guess what, that active kick you feel is for real! Most pregnant persons feel baby’s first kicks around weeks 16-20. Those earliest movements are often felt like “flutters” or “gas bubbles.”

Fetal movement will continue more frequently as your baby grows and everyone will marvel at baby’s movement inside your growing belly! Many pregnancy providers check in on baby with a mid-2nd trimester ultrasound. Caution: During this peek inside your womb, baby’s external genitalia may be seen or be obvious—if you want to know the sex of your baby, now is the time to ask. If you’re waiting for baby to emerge, let the ultrasound person and your pregnancy care provider know you want to wait to learn baby’s sex until after birth.

Your pregnancy care provider will check baby’s heart tones at each visit; and baby’s hearts beat, which is fast—between 120 to 160 beats per minute! Other milestones during this trimester include the emergence and growth of baby’s eyebrows and hair, nails, brain and muscles. By the end of this second trimester, your baby can respond to light and sound, and their eyelids can open and close.

Now is the time to schedule prenatal classes, such as a childbirth education class and breastfeeding class—these are typically held at the facility where you plan to give birth. If you’re having a home birth, you can still attend a private or local hospital-based prenatal class.

Your 3rd Trimester: Weeks 27-40

Baby now blossoms from a butternut squash to a large watermelon. You know this is happening as the size of your uterus has risen almost to your breastbone—creating discomfort with breathing, reflux and sleeping.

The aches and pains of baby pressing on your bladder and ligaments, and the restless nights, may make you yearn to get this wee one born. However, their lungs and brain are not completely developed until closer to 40 weeks of pregnancy needed for the best start in life.

This third trimester is all about preparing baby for life outside of your uterus. It’s important not to rush this time— in your third trimester baby’s brain triples in its weight and size!

This is an important time to pay attention to baby’s movement and activity. Your pregnancy care provider may ask you to monitor baby’s kick counts, which help you check on how well baby is doing inside your expanding belly. Decreased movement by your baby in your womb has been associated with developmental delays in children and even stillbirth.

Kick counts are easy; they just take a few minutes during which time you’re bonding with your baby. After a few sessions, you’ll get to know what’s normal—and not—for your little one. Find that time each day when baby seems most active. Stop what you’re doing, sit down, relax and pay attention to baby’s movements.

With your hands on your growing belly, start counting each flutter or move. You should feel at least 10 movements within 2 hours, say experts at the American College of Obstetricians and Gynecologists. Of course, most babies will have 10 movements in a much shorter time. In fact, it may only take about 30 minutes to get to 10 movements. Once you reach 10, you’re done!

If the kick counts decrease be sure to call your pregnancy provider. Other signs to watch for and alert your pregnancy provider to are:
  • If your “bag of waters” break (it can be a trickle or a gush)
  • You have bright red bleeding

When in doubt, check in with your pregnancy care provider, who can calm your fears or assess for a true emerging problem or emergency. You’ve got this mama, and in no time at all you’ll be ready to welcome baby to its home with you.

Joanne Goldbort, PhD, RN

Joanne Goldbort, PhD, RN, is an assistant professor and lead faculty for Maternal Child Nursing at Michigan State University.

The information contained on this article should not be used as a substitute for the medical care and advice of your health care professional.