The three stages of pregnancy are called trimesters, and each lasts roughly 13 to 14 weeks. The first trimester covers weeks 1 through 12, the second spans weeks 13 through 28, and the third runs from week 29 until delivery, typically around week 40. Each trimester brings distinct changes to both the developing baby and your body.
First Trimester: Weeks 1 Through 12
What’s Happening to the Baby
The first trimester is when the most dramatic transformation occurs. A single fertilized cell divides into millions, forming the foundation for every organ system. The neural tube, which becomes the brain and spinal cord, closes within the first several weeks. The heart begins beating and pumping blood. Tiny limb buds appear and develop into arms and legs. By around week 8, the developing baby transitions from being called an embryo to a fetus, meaning all major organ structures have started forming. By week 12, your baby is about the size of a lime.
This period carries the highest risk of miscarriage, which is why many people wait until the end of the first trimester to share their news. It’s also a window when the baby is most vulnerable to harmful exposures, since organs are actively forming.
What You May Feel
Hormonal changes affect almost every organ system in your body, and the first trimester is when those shifts hit hardest. A missed period is the most obvious early sign. After that, extreme tiredness, tender and swollen breasts, and nausea (often called morning sickness, though it can strike any time of day) are the hallmarks. Some people experience a more severe form of nausea and vomiting called hyperemesis gravidarum, which goes beyond typical morning sickness and may require treatment.
Common Screenings
Early prenatal visits establish a baseline for your health and the baby’s development. A blood-based screening called cell-free DNA testing can check for common chromosomal conditions like Down syndrome as early as the first trimester. The American College of Obstetricians and Gynecologists recommends this screening be made routinely available to all pregnant patients as the most sensitive and specific option for detecting trisomies 21, 18, and 13. You always have the right to pursue or decline any genetic screening after discussing it with your provider.
Second Trimester: Weeks 13 Through 28
What’s Happening to the Baby
With the major organs in place, the second trimester is about growth, refinement, and sensory development. Around week 15, bones become visible on ultrasound. By week 16, the eyes can move slowly, the ears are nearing their final position, and limb movements become coordinated enough to show up on imaging. By week 18, the baby may begin to hear sounds.
One of the most anticipated milestones happens around week 18 to 20: quickening, or the first time you feel the baby move. It often feels like fluttering or bubbling. By week 25, the baby may respond to familiar sounds like your voice, and rapid eye movements begin during sleep, a sign of maturing brain activity. At 27 weeks, the baby is roughly the size of a head of cauliflower.
What You May Feel
Many people consider the second trimester the most comfortable stretch. Nausea and fatigue from the first trimester typically fade. Your belly becomes visibly larger, and you may start to feel more energetic. That said, new symptoms can appear: back pain, round ligament pain in the lower abdomen, and skin changes like darkened patches on the face.
Key Tests in This Window
The anatomy scan ultrasound, usually performed around week 20, gives a detailed look at the baby’s organs, limbs, and overall growth. This is also the trimester when screening for gestational diabetes takes place, typically between weeks 24 and 28. Preeclampsia, a serious condition involving high blood pressure and protein in the urine, can develop after 20 weeks, so blood pressure monitoring becomes increasingly important from this point forward.
Third Trimester: Weeks 29 Through 40
What’s Happening to the Baby
By week 31, the baby has finished most major development and enters a phase of rapid weight gain. Fat accumulates under the skin, smoothing out the wrinkled appearance and helping the baby regulate temperature after birth. The lungs are among the last organs to fully mature, producing a substance that keeps the air sacs from collapsing once the baby starts breathing on its own.
By week 36, most babies have turned head down in preparation for delivery. If your baby hasn’t turned by around week 37, your provider may discuss techniques to encourage repositioning. At 40 weeks, a full-term baby is about the size of a watermelon.
What You May Feel
The third trimester is physically the most demanding. The growing baby puts pressure on your bladder, lungs, and lower back. Frequent urination, shortness of breath, swelling in the feet and ankles, and difficulty sleeping are all common. Braxton Hicks contractions, irregular tightening of the uterus that isn’t true labor, may start occurring more often. Many people also feel the baby’s movements more intensely, including kicks and rolls that are visible from the outside.
Monitoring and Complications to Watch For
Prenatal visits become more frequent in the third trimester, often every two weeks and then weekly as your due date approaches. Preeclampsia remains a concern, with gestational hypertension (high blood pressure that develops after 20 weeks) also on the radar. Signs like sudden severe headaches, vision changes, or rapid swelling warrant prompt attention. If you’re at high risk for preeclampsia, low-dose aspirin may be recommended after 12 weeks of pregnancy as a preventive measure.
Weight Gain Across All Three Stages
Total recommended weight gain depends on your pre-pregnancy BMI. For someone starting at a normal weight (BMI of 18.5 to 24.9), the target is 25 to 35 pounds over the entire pregnancy. For those who are underweight, it’s 28 to 40 pounds. If you’re overweight, 15 to 25 pounds is the range, and for those with a BMI of 30 or higher, 11 to 20 pounds. Twin pregnancies call for significantly more: 37 to 54 pounds for someone starting at a normal weight.
Most of the weight gain happens in the second and third trimesters as the baby, placenta, amniotic fluid, and your own blood volume all increase rapidly. First-trimester gain is minimal for most people, especially if nausea limits appetite.
The “Fourth Trimester”
Though not one of the three official stages, the 12 weeks after delivery are increasingly recognized as a critical phase sometimes called the fourth trimester. Your body is recovering from birth, hormones are shifting dramatically again, and the demands of caring for a newborn are intense. This period carries real medical significance: maternal mortality is highest in the first 42 days after delivery, accounting for 45% of total pregnancy-related deaths. Postpartum checkups during this window are just as important as the prenatal visits that came before.