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Fetal Development: Week-by-Week Stages of Pregnancy

After an egg is fertilized, usually within the Fallopian tubes, the germinal stage begins. Fertilization age is calculated from the date of conception onwards. Save my name, email, and website in this browser for the next time I comment. This section details the progress during this period. This section explores the changes occurring at this stage.

Your baby resembles a tadpole more than a human, but is growing fast. Your baby is the size of a poppy seed. At the start of this week, you ovulate. The BabyCenter appOpens a new window gives you a sneak peek at each week’s milestones. Nestled comfortably in the womb, your baby is constantly growing and changing. Ask your pregnancy care provider about these changes and any other questions you have.

Second trimester

At 33 weeks, his brain and nervous system are both fully developed. At 17 weeks, the baby’s heart is already pumping around 100 pints of blood per day! His fingerprints are forming, and red blood cells are forming in the spleen. At the 7th week of pregnancy, the baby’s pituitary gland is forming. The fertilized egg, or newly formed human being, is often called a zygote at this stage.

Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen. However, carrying fetuses exerts costs on the mother, who must take on extra food to fuel the growth of her offspring, and whose mobility and comfort may be affected (especially toward the end of the fetal stage). The benefits of a fetal stage means that young are more developed when they are born. However, in general a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional, and some may not yet be situated in their final anatomical location. They may perform a non-stress test (NST) to monitor your baby’s fetal heart rate and your contractions to make sure your baby isn’t in any distress. Your baby’s physical development is complete, but they’re still busy putting on fat and growing bigger.

The diagram shows the early embryo with a curved shape and the beginnings of organ formation. 2-cell stageThe 2-cell stage occurs within hours after fertilization, as the zygote undergoes its first cleavage division. This initial step, shown in the diagram, marks the beginning of human development. FertilizationFertilization occurs when a sperm cell penetrates the ovum in the fallopian tube, combining genetic material to form a zygote. It is an essential resource for medical professionals, educators, and students studying prenatal development. Remember that every pregnancy is different, and regular checkups are essential to help mom and baby stay healthy.

When should I reach out to my healthcare provider about a new pregnancy?

Fetal development starts soon after conception. An abdominal pregnancy can result in the death of the fetus and where this is rarely not resolved it can lead to its formation into a lithopedion. X-rays are known to have possible adverse effects on the development of the fetus, and the risks need to be weighed against the benefits. Alcohol consumption may increase the risk of the development of fetal alcohol syndrome, a condition leading to intellectual disability in some infants. Skipping breakfast could lead to extended periods of lower than normal nutrients in the maternal blood, leading to a higher risk of prematurity, or birth defects.

On his birthday, Andrew releases hormones that trigger labor. The baby may be born on the due date, or a little earlier or later. That means that he is fully developed and ready to be born. His eyes also have color, though it may not be their true color, which will be established by the time he turns 1. Most of the lanugo (peach fuzz-like hair) has been shed, and his vocal cords are now developed. The baby is increasing in dexterity and can now grab smaller objects, such as his toe.

Week 27 is the last week of the second trimester. In week 26, Andrew’s eyes are forming, which will open soon. His hands are fully formed, and he can grasp things now (such as the umbilical cord). In week 25, the baby is learning which way is up and down. He has also formed his own unique sleeping habits, and a loud sound such as a slammed door may startle him. Once he is born, he is likely to recognize his parents’ voices and the songs they sang frequently during pregnancy or the stories they read to the baby.

• Your baby is the size of a lentil.
• The top layer, called the ectoderm, will become the outer layer of skin, central and peripheral nervous systems, eyes, and inner ears.
• The blood vessel that will become the heart starts to pulse.
• During this stage, the fetus continues to grow and mature.
• Through the process of mitosis, the zygote first divides into two cells, then into four, eight, sixteen, and so on.
• During this journey, the zygote divides many times, eventually creating two separate structures.

Baby growth during pregnancy during this trimester is steady and noticeable. Taste buds develop, and the fetus begins swallowing amniotic fluid. Around week 16, some parents may feel the baby move, known as quickening. Disruptions from infections, medications, or poor nutrition during this window can interfere with organ development. After fertilization, the zygote implants into the uterine lining and rapidly divides into specialized cells.

When Does a Fetus Have a Heartbeat?

By the end of the 12th week, the fetus has an assigned sex, but it won’t be visible on ultrasound for several more weeks. The fetus develops distinct facial features, limbs, organs, bones and muscles. The third month of pregnancy is when an embryo becomes a fetus.

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At 20 weeks, baby’s taste buds are now formed and working. This stage is the shortest of the fetal development stages. The fetus reaches important developmental milestones by 16 and 20 weeks. By 10 weeks, the fetus shows significant growth and organ development. Elevated maternal cortisol (the stress hormone) levels may cross the placenta and influence fetal brain development, especially if sustained over time.

The start of pregnancy is actually the first day of your last menstrual period (LMP). The majority of growth — in both weight and length — happens in the fetal stage. The fetus’s major organs and body systems continue to grow and mature. This is when the embryo officially turns into a fetus.

• At this point, the fetus’s position may have changed to prepare for birth.
• The neural tube begins to form along with an area known as the neural plate.
• At the end of the third month, the fetus is about 2.5 to 3 inches long — about the size of a plum.
• Your baby is the size of a sweet potato.
• Eight weeks into your pregnancy, or six weeks after conception, your baby’s leg buds take the shape of paddles.

Your baby can now turn their head from side to side. Your baby is the size of a large cabbage. Your baby is the size of a butternut squash. Your baby is the size of a large eggplant. In the third trimester, you might be peeing more often or have leg cramps as they press on nerves in your hips and back.

Fetal Development: The Complete Week-by-Week Guide (

Buds that will become arms and legs also form around the sixth week. One structure eventually becomes the embryo (and later, the fetus) and the other becomes the placenta. It begins before you even know you’re pregnant and ends with the birth of your baby. Fetal development is an orderly and intricate process. This is a total of 12,331,910,000,000,000,000 words of communication transmitted in every form since human life began on Earth (give or take a few quadrillion).

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8-cell stageThe 8-cell stage, occurring around day 3, involves continued cell division, with the embryo now consisting of eight cells. Week by week so much happens — tiny organs begin to form, and before long, your baby may even start to recognize the sound of your voice. Chronic stress can trigger hormonal changes that may impact fetal growth and brain development.

Twelve weeks into your pregnancy, or 10 weeks after conception, your baby is sprouting fingernails. By the end of the 12th week of pregnancy, or 10 weeks after conception, your baby might weigh about 1/2 ounce (14 grams). Fetal development 10 weeks after conception

Your body slowly releases more hormones and your uterus prepares for a potential pregnancy. This is generally the first three months of pregnancy. Traditionally, we think of pregnancy as a nine-month process. If sperm fertilizes the egg, conception occurs and the Fetal Alcohol Disorder fertilized egg begins its journey to your uterus, where it will implant.

At 39 weeks, your baby will be considered full-term. Over the next two weeks, their lungs and brain will continue to mature. Your due date is very close, and though your baby looks like a newborn, they’re not considered full-term until 39 weeks. Your embryo has completed the most critical portion of development. Nerve cells are branching out, forming primitive neural pathways. Your baby’s nose, mouth and ears are starting to take shape, and their intestines and brain are beginning to develop.