Fertilized Egg !!TOP!!
If a sperm is successful on its quest to fertilize an egg, the now fertilized egg (called a zygote) continues to move down your fallopian tube, dividing into two cells, then four cells, then more cells. About a week after the sperm has fertilized the egg, the zygote has traveled to your uterus. It's now a growing cluster of about 100 cells called a blastocyst.
The blastocyst then attaches itself to the lining of your uterus (the endometrium). This attachment process is called implantation. However, just because conception occurs doesn't mean implantation will. Sometimes implantation doesn't happen, and you pass the fertilized egg in your next menstrual period.
Not usually. You may notice signs that you've ovulated, such as changes in your cervical mucus or basal body temperature. However, most people don't feel fertilization. You may feel a dull ache or experience light spotting several days after conception. This could be from the fertilized egg implanting in your uterus.
The question of when life begins is an eternal one, debated by philosophers and theologians for centuries, and likely destined to forever elude consensus. However, on the separate but closely related question of when a woman is considered pregnant, the medical community has long been clear: Pregnancy is established when a fertilized egg has been implanted in the wall of a woman's uterus. The definition is critical to distinguishing between a contraceptive that prevents pregnancy and an abortifacient that terminates it. And on this point, federal policy has long been both consistent and in accord with the scientists: Drugs and devices that act before implantation prevent, rather than terminate, pregnancy.
To be sure, not every act of intercourse results in a pregnancy. First, ovulation (i.e., the monthly release of a woman's egg) must occur. Then, the egg must be fertilized. Fertilization describes the process by which a single sperm gradually penetrates the layers of an egg to form a new cell ("zygote"). This usually occurs in the fallopian tubes and can take up to 24 hours. There is only a short window during which an egg can be fertilized. If fertilization does not occur during that time, the egg dissolves and then hormonal changes trigger menstruation; however, if fertilization does occur, the zygote divides and differentiates into a "preembryo" while being carried down the fallopian tube toward the uterus. Implantation of the preembryo in the uterine lining begins about five days after fertilization. Implantation can be completed as early as eight days or as late as 18 days after fertilization, but usually takes about 14 days. Between one-third and one-half of all fertilized eggs never fully implant. A pregnancy is considered to be established only after implantation is complete.
This is clearly a cause for discomfort within the ranks of the abortion opponents. Some groups, notably including the National Right to Life Committee, try to avoid the issue entirely, saying they have no position on contraception. But many, including Concerned Women for America and the Secretariat for Pro-Life Activities of the U.S. Conference of Catholic Bishops, are clear and consistent: For them, pregnancy begins at fertilization, and if that "fact" implicates contraception, so be it. As far back as 1981, Judie Brown, long-time president of the American Life League, made the point quite clear in testimony before a congressional committee: "However, once a chemical or device acts to destroy the newly fertilized egg, which is a brand new life, then we are not any longer dealing with a contraceptive. We're dealing with an abortion."
The ongoing debate over emergency contraception has put the question of the dividing line between preventing and disrupting pregnancy back in the public eye. A product packaged specifically to be used as emergency contraception was first approved by the FDA in 1998 as a method of pregnancy prevention; the agency approved a second such product, Plan B, a year later. In a question-and-answer document developed in 2004, the FDA was explicit in describing the drug's method of action: "Plan B works like other birth control pills to prevent pregnancy. Plan B acts primarily by stopping the release of an egg from the ovary (ovulation). It may prevent the union of sperm and egg (fertilization). If fertilization does occur, Plan B may prevent a fertilized egg from attaching to the womb (implantation)." In short, despite the confusion that opponents have fostered surrounding emergency contraception's mode of action, how the method works depends more on when during a woman's monthly menstrual cycle it is taken (and, specifically, whether she has ovulated) than on when she had sexual intercourse.
The fertilized egg starts growing fast, dividing into many cells. It leaves the fallopian tube and enters the uterus 3 to 4 days after fertilization. In rare cases, the fertilized egg attaches to the fallopian tube. This is called a tubal pregnancy or ectopic pregnancy and is a danger to the mother.
Pregnancy begins with a fertilized egg. Normally, the fertilized egg attaches to the lining of the uterus. An ectopic pregnancy occurs when a fertilized egg implants and grows outside the main cavity of the uterus.
If the fertilized egg continues to grow in the fallopian tube, it can cause the tube to rupture. Heavy bleeding inside the abdomen is likely. Symptoms of this life-threatening event include extreme lightheadedness, fainting and shock.
During in vitro fertilization, eggs are removed from mature follicles within an ovary (A). An egg is fertilized by injecting a single sperm into the egg or mixing the egg with sperm in a petri dish (B). The fertilized egg (embryo) is transferred into the uterus (C).
During IVF, mature eggs are collected (retrieved) from ovaries and fertilized by sperm in a lab. Then the fertilized egg (embryo) or eggs (embryos) are transferred to a uterus. One full cycle of IVF takes about three weeks. Sometimes these steps are split into different parts and the process can take longer.
Women who don't have a functional uterus or for whom pregnancy poses a serious health risk might choose IVF using another person to carry the pregnancy (gestational carrier). In this case, the woman's eggs are fertilized with sperm, but the resulting embryos are placed in the gestational carrier's uterus.
If a sperm penetrates the egg, fertilization results. Tiny hairlike cilia lining the fallopian tube propel the fertilized egg (zygote) through the tube toward the uterus. The cells of the zygote divide repeatedly as the zygote moves down the fallopian tube to the uterus. The zygote enters the uterus in 3 to 5 days.
If more than one egg is released and fertilized, the pregnancy involves more than one fetus, usually two (twins). Because the genetic material in each egg and in each sperm is slightly different, each fertilized egg is different. The resulting twins are thus fraternal twins. Identical twins result when one fertilized egg separates into two embryos after it has begun to divide. 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.
In order for pregnancy to happen, sperm needs to meet up with an egg. Pregnancy officially starts when a fertilized egg implants in the lining of the uterus. It takes up to 2-3 weeks after sex for pregnancy to happen.
There are 2 ways that twins can happen. Identical twins are made when 1 already-fertilized egg splits into 2 separate embryos. Because identical twins come from the same sperm and egg, they have the same genetic material (DNA) and look exactly alike.
The whole developmental program, from the first division of the fertilized egg to the birth of a fully formed organism consisting of trillions of cells, is largely a diversification of the transcription factor collections in cells as they divide. How does a cell that is destined to contribute to the iris of the eye come to possess just the right set of transcription factors to ensure that the genes for making an iris (and not genes for making a retina, or lens, or cornea) are expressed at just the right levels and at just the right time as the embryo develops? In other words, how do different cells come to possess different transcription factors?
The genes that encode transcription factors, like all genes, have rheostats that govern their output, and, as is the case for all genes, those rheostats are set by transcription factors. So the set of transcription factors that a given cell has at any given moment is the result of which particular transcription factor genes were expressed during the course of development of that cell. This logic makes the developing organism seem like a set of nested Russian dolls: to determine why a set of transcription factors came to be present in a liver cell, you have to look at the transcription factors in the cell that gave rise to the liver cell, and to determine why that particular set of transcription factors came to be present in that cell you have to look at its precursor cell, and so on, all the way back to the original fertilized egg.
A concentration gradient of a single transcription factor will already define three zones of the fertilized egg: a zone of high concentration at one end of the egg (say, where the head of the fly will form), where the factor turns on genes containing strong and weak recognition sequences for the transcription factor; a zone of medium concentration near the middle of the egg, where it turns on only genes that have close matches to the recognition sequence (strong binding sites for the transcription factor); and a zone of low concentration near the opposite end of the egg (where the tail of the fly will form), where there is not enough of the transcription factor to turn on either kind of gene (see figure). 041b061a72