When is a zygote a fetus

Because one egg was fertilized by one sperm, the genetic material in the two embryos is the same. Once a month, an egg is released from an ovary into a fallopian tube. After sexual intercourse, sperm move from the vagina through the cervix and uterus to the fallopian tubes, where one sperm fertilizes the egg.

The fertilized egg zygote divides repeatedly as it moves down the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells. Then it becomes a hollow ball of cells called a blastocyst. Inside the uterus, the blastocyst implants in the wall of the uterus, where it develops into an embryo attached to a placenta and surrounded by fluid-filled membranes.

About 6 days after fertilization, the blastocyst attaches to the lining of the uterus, usually near the top. This process, called implantation, is completed by day 9 or The wall of the blastocyst is one cell thick except in one area, where it is three to four cells thick. The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta.

The placenta produces several hormones that help maintain the pregnancy. For example, the placenta produces human chorionic gonadotropin, which prevents the ovaries from releasing eggs and stimulates the ovaries to produce estrogen and progesterone continuously.

The placenta also carries oxygen and nutrients from mother to fetus and waste materials from fetus to mother. Some of the cells from the placenta develop into an outer layer of membranes chorion around the developing blastocyst.

Other cells develop into an inner layer of membranes amnion , which form the amniotic sac. When the sac is formed by about day 10 to 12 , the blastocyst is considered an embryo.

The amniotic sac fills with a clear liquid amniotic fluid and expands to envelop the developing embryo, which floats within it. The next stage in development is the embryo, which develops within the amniotic sac, under the lining of the uterus on one side.

This stage is characterized by the formation of most internal organs and external body structures. Most organs begin to form about 3 weeks after fertilization, which equals 5 weeks of pregnancy because doctors date pregnancy from the first day of the woman's last menstrual period, which is typically 2 weeks before fertilization. At this time, the embryo elongates, first suggesting a human shape. Shortly thereafter, the area that will become the brain and spinal cord neural tube begins to develop.

The heart and major blood vessels begin to develop earlier—by about day The heart begins to pump fluid through blood vessels by day 20, and the first red blood cells appear the next day. Blood vessels continue to develop in the embryo and placenta. Almost all organs are completely formed by about 10 weeks after fertilization which equals 12 weeks of pregnancy. The exceptions are the brain and spinal cord, which continue to form and develop throughout pregnancy. Most malformations birth defects occur during the period when organs are forming.

During this period, the embryo is most vulnerable to the effects of drugs, radiation, and viruses. At 8 weeks of pregnancy, the placenta and fetus have been developing for 6 weeks. The placenta forms tiny hairlike projections villi that extend into the wall of the uterus.

The cervical mucus acts as a reservoir for extended sperm survival. Once the sperm have entered the uterus, contractions propel the sperm upward into the fallopian tubes. The first sperm enter the tubes minutes after ejaculation. The first sperm, however, are likely not the fertilizing sperm.

Motile sperm can survive in the female reproductive tract for up to five days. Egg transport begins at ovulation and ends once the egg reaches the uterus. Following ovulation, the fimbriated, or finger-like, end of the fallopian tube sweeps over the ovary.

Adhesive sites on the cilia, which are located on the surface of the fimbriae, are responsible for egg pickup and movement into the tube. The cilia within the tube, and muscular contractions resulting from the movement of the egg, create a forward motion. Transport through the tube takes about 30 hours. Conditions such as pelvic infections and endometriosis can permanently impair the function of the fallopian tubes, due to scarring or damage to the fimbriae.

Following ovulation, the egg is capable of fertilization for only 12 to 24 hours. Contact between the egg and sperm is random.

Once the egg arrives at a specific portion of the tube, called the ampullar-isthmic junction, it rests for another 30 hours. Fertilization — sperm union with the egg — occurs in this portion of the tube. The fertilized egg then begins a rapid descent to the uterus. The period of rest in the tube appears to be necessary for full development of the fertilized egg and for the uterus to prepare to receive the egg. Defects in the fallopian tube may impair transport and increase the risk of a tubal pregnancy, also called ectopic pregnancy.

A membrane surrounding the egg, called the zona pellucida, has two major functions in fertilization. First, the zona pellucida contains sperm receptors that are specific for human sperm. Second, once penetrated by the sperm, the membrane becomes impermeable to penetration by other sperm. Following penetration, a series of events set the stage for the first cell division.

The single-cell embryo is called a zygote. Over the course of the next seven days, the human embryo undergoes multiple cell divisions in a process called mitosis. At the end of this transition period, the embryo becomes a mass of very organized cells, called a blastocyst.

It's now believed that as women get older, this process of early embryo development is increasingly impaired due to diminishing egg quality. Once the embryo reaches the blastocyst stage, approximately five to six days after fertilization, it hatches out of its zona pellucida and begins the process of implantation in the uterus. Editorial team. Fetal development.

Learn how your baby is conceived and how your baby develops inside the mother's womb. Week 1 to 2 The first week of pregnancy starts with the first day of a woman's menstrual period. She is not yet pregnant. During the end of the second week, an egg is released from an ovary.

This is when you are most likely to conceive if you have unprotected intercourse. Week 3 During intercourse, sperm enters the vagina after the man ejaculates. The strongest sperm will travel through the cervix the opening of the womb, or uterus , and into the fallopian tubes.

A single sperm and the mother's egg cell meet in the fallopian tube. When the single sperm enters the egg, conception occurs. The combined sperm and egg is called a zygote. The zygote contains all of the genetic information DNA needed to become a baby.

Half the DNA comes from the mother's egg and half from the father's sperm.