When is a zygote an embryo

At the end of the 8th week after fertilization 10 weeks of pregnancy , the embryo is considered a fetus. During this stage, the structures that have already formed grow and develop. The following are markers during pregnancy:. By about 16 to 20 weeks: Typically, the pregnant woman can feel the fetus moving. Women who have been pregnant before typically feel movements about 2 weeks earlier than women who are pregnant for the first time.

The lungs continue to mature until near the time of delivery. The brain accumulates new cells throughout pregnancy and the first year of life after birth. As the placenta develops, it extends tiny hairlike projections villi into the wall of the uterus.

The projections branch and rebranch in a complicated treelike arrangement. This arrangement greatly increases the area of contact between the wall of the uterus and the placenta, so that more nutrients and waste materials can be exchanged. The placenta is fully formed by 18 to 20 weeks but continues to grow throughout pregnancy. At delivery, it weighs about 1 pound.

Merck and Co. From developing new therapies that treat and prevent disease to helping people in need, we are committed to improving health and well-being around the world. The Manual was first published in as a service to the community. Learn more about our commitment to Global Medical Knowledge. This site complies with the HONcode standard for trustworthy health information: verify here. Common Health Topics. Development of the Blastocyst. Development of the Embryo.

Development of the Fetus and Placenta. Normal Pregnancy. Test your knowledge. Postpartum depression is a feeling of extreme sadness and related psychologic disturbances after delivery of a baby.

Although it is common to feel sad or miserable within 3 days of delivery the baby blues , these feelings usually go away within 2 weeks. Postpartum depression is a more serious mood change. Women in which of the following groups are at increased risk for this condition? More Content. Click here for the Professional Version. From Egg to Embryo Once a month, an egg is released from an ovary into a fallopian tube. Placenta and Embryo at About 8 Weeks At 8 weeks of pregnancy, the placenta and fetus have been developing for 6 weeks.

Allows materials to be exchanged between the blood of the mother and that of the embryo. The embryo floats in fluid amniotic fluid , which is contained in a sac amniotic sac. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. The different cells derived from cleavage up to the blastula stage are called blastomeres.

Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic total or meroblastic partial. Cell cleavage : Early development is characterized by cleavage of the zygote, which refers to cell divisions that are not associated with significant growth of the embryo.

Holoblastic cleavage occurs in animals with little yolk in their eggs. These species, such as humans and other mammals, receive nourishment as embryos from the mother via the placenta or milk after birth.

On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk, such as birds and oviparous reptiles although some viviparous reptiles also exist. Since cleavage is impeded by the vegetal pole, there is a very uneven distribution and size of cells.

Cells are more numerous and smaller at the animal pole of the zygote than at the vegetal pole. In holoblastic eggs, the first cleavage always occurs along the vegetal—animal axis of the egg, and the second cleavage is perpendicular to the first. From here, the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage in various organisms.

The end of cleavage is known as the midblastula transition and coincides with the onset of zygotic transcription. In amniotes, the cells of the morula are at first closely aggregated. However, they quickly become arranged into an outer or peripheral layer, the trophoblast, and an inner cell mass.

The trophoblast does not contribute to the formation of the embryo proper; the embryo develops from the inner cell mass. Fluid collects between the trophoblast and the greater part of the inner cell mass, and thus the morula, is converted into the blastodermic vesicle also called the blastocyst or blastula. The inner cell mass remains in contact with the trophoblast at one pole of the ovum.

This is named the embryonic pole, since it indicates the location where the future embryo will develop. In the case of monozygotic twins derived from one zygote , a zygote divides into two separate cells embryos at the first cleavage division. Monozygotic twins can also develop from two inner cell masses. A rare occurrence is the division of a single inner cells mass giving rise to twins. However, if one inner cell mass divides incompletely, the result is conjoined twins.

Dizygotic twins is the development of two embryos from two different zygotes. The blastocyst forms early in embryonic development and has two layers that form the embryo and placenta. In humans, the blastocyst is formed approximatelyy five days after fertilization. This stage is preceded by the morula.

The morula is a solid ball of about 16 undifferentiated, spherical cells. As cell division continues in the morula, the blastomeres change their shape and tightly align themselves against each other.

This is called compaction and is likely mediated by cell surface adhesion glycoproteins. The blastocyst possesses an inner cell mass ICM , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta.

The trophoblast surrounds the inner cell mass and a fluid-filled, blastocyst cavity known as the blastocoele or the blastocystic cavity. The embryoblast is the source of embryonic stem cells and gives rise to all later structures of the adult organism. The trophoblast combines with the maternal endometrium to form the placenta in eutherian mammals. Blastocyst : The blastocyst possesses an inner cell mass from which the embryo will develop, and an outer layer of cells, called the trophoblast, which will eventually form the placenta.

Before gastrulation, the cells of the trophoblast become differentiated into two strata. The outer stratum forms a syncytium, which is a layer of protoplasm studded with nuclei that shows no evidence of subdivision into cells termed the syncytiotrophoblast.

Those buds of arms and legs have turned into paddles. Your baby is still as tiny as a pencil eraser, but they already have little nostrils. The lenses of their eyes are beginning to form. Their upper lip and nose are also starting to take shape. Their toes are forming, too. Their eyelids and ears are getting more refined. Your baby started as a tiny speck and is still less than 2 inches long from crown to rump.

Still, your little one is starting to look like a tiny newborn. From week 11 onward, your baby will continue to develop and grow until the end of your pregnancy. Their face has taken on more human characteristics. Week 13 marks the start of the second trimester. During this stage, your fetus is looking and operating more like a real baby.

Early on, their sex organs are developing, their bones are getting stronger, and fat is starting to accumulate on their body. Midway through, their hair becomes visible, and they can suck and swallow. They can start to hear your voice, too. In the first half of this stage, your fetus starts to open their eyes, practices breathing in amniotic fluid, and becomes covered in vernix caseosa.

Toward the end, they are gaining weight more quickly, making lots of big movements, and starting to crowd themself in the amniotic sac.

Your fetus starts the third trimester at 10 inches from crown to rump, and grows to 18 to 20 inches. The length and weight of babies at delivery varies greatly. Early pregnancy can be difficult on your mind and emotions.

Researchers estimate that between 10 to 25 percent of all clinically recognized pregnancies end in miscarriage pregnancy loss before 20 weeks. The rest usually happen before week