The duration of pregnancy is calculated from the date of the last menstrual period to the estimated due date, typically spanning 280 days or 40 weeks. However, depending on the pregnancy, the number of weeks may vary slightly. Gestation divides into trimesters, and here, we’ll delve into the key weeks when significant milestones or events occur:

First trimester

Week 0 to 2:

The journey of life begins. After the miraculous union of egg and sperm, the zygote rapidly divides, transforming into a morula. As it embarks on its journey through the fallopian tubes to the uterus, it blossoms into a blastocyst, releasing the embryo as its outer membrane breaks. The embryo then nestles into the uterus, finding its home in the future placenta, a structure that will nourish and protect it throughout its development.

Week 3 to 8:

The embryo is already attached to the uterine wall, which develops a tissue rich in essential nutrients for growth. Around week 5, the baby’s central systems, including the nervous system, begin to form from the neural tube. The limbs, skeletal system, and several nerves develop, as well as the eyes and ears. The heart is already functioning properly, blood circulates, and the lungs begin to mature, although they are not yet functional. The embryo makes involuntary movements coordinated by its nerve units and not yet perceptible to the mother.

Week 9 to 12:

The baby’s main organs are forming, and some already work. Elbows, fingers, hair follicles, and nipples develop. The embryo’s face acquires a more human shape, and all its sensory organs are visible. Between week 10 and week 11, the baby’s heartbeat is audible through Doppler, and the placenta works correctly. By week 12, the small intestine protrudes from the baby's abdomen; it can already swallow the amniotic liquid and eliminate waste through urine, creating a closed nutritional cycle. The pregnant woman must maintain a balanced diet, as the baby requires more energy.

Second trimester

Week 13 to 15

he baby begins to perceive light and can move away from luminous stimuli. Cartilaginous tissue begins calcifying, and the nose, ribs, spine, and leg bones grow. The digestive system is already placed in the abdominal cavity and works. The baby’s skin is thin, and a layer of hair (lanugo) begins to form, covering the body, eyebrows, and a small mustache. Red blood cells and nerve cells are functioning, the brain has two hemispheres, and the baby develops more coordination in gestures and movements. The body is more proportional, the head is round, and the neck is elongated, as are the limbs. The baby can make fists and nails, and some tooth buds begin to appear.

Week 16 to 18

The baby’s moves are now perceivable by the pregnant woman, and the brain controls them altogether; the baby puts their hands in their mouth, sucks, yawns, and opens or closes their mouth. Taste buds develop, and the pancreas and liver secrete hormones. The eyes develop the lens and cornea. The baby's head looks more upright, the grasp reflex appears, and fingerprints form. The ears are in their place, and the baby hears sounds like the woman’s heartbeat and people’s voices. Their reproductive organs are visible, meconium production (first feces) begins, and the baby and the pregnant woman show increased visibility of the veins due to vascularization. Estrogen and progesterone levels rise in the pregnant woman, so she produces colostrum, and her skin darkens in areas such as the nipples and linea Alba. Mild cognitive changes may occur.

Week 19 to 24

The baby’s ears fully develop and respond to sounds. They have a stronger heartbeat now audible via stethoscope. Bone marrow produces red blood cells, subdermal fat accumulates, the skin thickens, and vernix (a waxy coating that protects the baby's skin from the amniotic fluid) forms. The brain rapidly develops, sharpening senses, accelerating growth, and perceiving maternal emotions. Taste buds extend to the cheeks, permanent teeth begin forming, and the face develops eyebrows, eyelashes, and a bony nose. The pregnant woman should monitor for preeclampsia or gestational diabetes, and specialists can now rule out congenital malformations.

Third Trimester

Week 25 to 28

During this term, the pregnant woman's body prioritizes providing nutrients to the baby and experiences Braxton-Hicks contractions, which, though not painful, may cause discomfort. The baby alternates between inactivity and noticeable movements, which can feel uncomfortable. Brain cells specialize, senses sharpen, and the fully developed eyes prepare to open. The pregnant woman's size reflects rapid growth and weight gain from body fat. While the baby’s organs function, they continue refining—the surfactant hormone peaks, aiding respiratory development. In boys, the testicles descend to the scrotum; in girls, the ovaries settle in the lower abdomen.

Week 29 to 35

The baby's vascular system is active, and they can now open and close their eyes. Their nervous system matures further, and reflex movements like finger and toe flexion appear. Preparing for birth, the baby’s soft bones and separated skull form the fontanelle. Hormonal production increases, the mother’s breasts grow, and may release colostrum. The baby stores essential minerals like iron, calcium, and phosphorus for organ and bone development. Their skin softens as fat deposits increase, their head becomes more proportionate, and short sleep cycles emerge. The baby can dilate pupils, develop sucking and swallowing patterns, expel urine, and accumulate meconium while their senses and immune system strengthen with placental nutrients.

Weeks 36 to 40

The pregnancy reaches full term during this period. The baby’s body is complete, and their organs are functional for life outside the womb. Their head is rounder, their hair more noticeable, the lanugo has almost disappeared, and their skin is thicker and reddish. While movements are less vigorous due to limited space, the baby remains active, can distinguish light variations, and responds more to touch and the mother’s movements. The placenta begins to calcify, reducing nutrient and waste exchange. Meconium has accumulated in the digestive system, and the baby will soon feel the need to expel it, triggering hormonal cycles that prepare both mother and baby for birth. The baby’s heart readies for independent function, ensuring oxygenation of the lungs and brain after delivery. Fully developed, the baby is ready to be born.