Weekly development of the embryo/fetus
Week 1 (Development of the embryo)
Ovulation (the release of the mature ovum from the ovary) happens approximately on the 14th day of the cycle, approximately 3-4 days in this period the conception may occur when the spermatocyte penetrates the lining of the oocyte and the cells fuse forming the zygote in the Fallopian tube. Shortly after the fertilization the zygote starts to divide rapidly into small cells known as blastomeres. 12-32 blastomeres form the morula which enters the uterus and attachés to its lining (the process known as the implantation).
The implantation finishes during the second week of the development. At this time occurs the bilaminar embrionic disc (namely, the embryo) which has two germ layers. The amnion, amnionic cavity, yolk sac and chorionic sac are being formed.
During the third week of development takes place gastrulation during which the bilaminar embryonic disc is transformed into trilaminar embryonic disc – the tree germ layers (ectoderm, endoderm and mesoderm) later give rise to all the tissues and organs of the human body. The vertebral column, central nervous system and the heart begin to develop. Now the embryo is about 1,5-3,0 mm in length.
At the beginning of the fourth week the embryo is almost straight. The embryo grows very quickly with folding occurring at the cranial and caudal ends simultaneously, which results in a cylindrical form of the embryo and it becomes slightly curved. The early heart starts to pump blood. At 28 days the upper limbs appear as small swelling-like buds. By the end of the fourth week the lower limb buds are identified. The length of the embryo reaches 4.0-6.0 mm.
The development of the brain and the facial part of the head causes the prevailing growth of the head in comparison to the rest of the embryo. The hand plates are formed. The embryo’s size is about 7.0-9.0 mm.
The embryo shows spontaneous movements, for example, twitching of the trunk and limbs, and responses reflectively to touch. The fingers start to develop on the hand plates. The upper limbs start to develop a few days earlier than the lower limbs.
The limbs change considerably during the seventh week. Lungs begin to develop. Upper limbs are longer than the lower.
The length of the fetus is approximately 16.0-18.0 mm.
For the first time occur coordinated limb movements. The bone ossification starts. By the end of the eighth week the fingers are separated.
The embryo represents visually distinct human characteristics, although the rounded head constitutes almost the half of the body. External genitalia already exists, but are not distinctive enough to identify the sex.
The length of the embryo is approximately 27.0-31.0 mm.
Since the ninth week the conceptus is called the fetus and so the fetal period lasts from the ninth week until the labor. The face of the fetus appears broad, the eyes are widely separated with the fused eyelids. The kidneys start producing urine between the ninth and the twelfth weeks, urine is discharge into the amnionic fluid which surrounds the fetus. The external genitalia are not developed until the end of the twelfth week, when the sex of the fetus may be distinguished.
The fetus’ weight is about 8 g at nine weeks and reaches 45 g until the end of the twelfth week.
The fetus grows rapidly during this period, especially the limbs. By the fourteenth week the movements become coordinated, although they are not felt by the mother as the movements are very slight. At the same time occur slow eye movements.
The weight of the fetus is about 110-200 g.
Now the mother feels the movements of the fetus. The skin is covered with a greasy material known as vernix caseosa which consists of dead epidermal cells and fatty secretions. Fine hair called lanugo covers the fetus completely.
By the end of the twentieth week the fetal weight is about 460 g.
The skin of the fetus is wrinkled and red as the capillaries lie close to the skin surface. During this period the fetus gains weight substantially. By 24 week fingernails appear and the lungs develop gradually.
Week 26- 29
Eyes of the fetus are open. Eyelashes are also present. The weight of the fetus is about 1500 g by the end of the week 29.
Skin of the fetus is smooth and pink. Lanugo is almost absent by the end of the week 36 and the fetus appears somewhat plump. The weight of the fetus close to the due date is about 3000-3500 g.
Expected date of delivery: Development of the embryo
The due date is approximately 280 days (40 weeks) after the first day of the last normal menstrual period or 266 days (38 weeks) after the conception as the fertilization occurs 2 weeks after the menstruation when ovulation happens.
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