For an egg (ovum) to be fertilized, it must find sperm waiting in the Fallopian tube. For the sperm to have time to traverse to the Fallopian tube, it must be deposited well in advance of ovulation.

Fertilization in vitro (in a tube) enables or improves the probability of the encounter between the ovule and the spermatozoon. In vivo (in the body), this encounter takes place in the Fallopian tube. Indeed, in vitro fertilization was devised to circumvent problems posed by tubal obstruction. Its range was later extended to other causes of subfertility in women and to problems of male subfertility, shedding light on the performance of spermatozoa. See similar questions...

BSE was first observed in Great Britain in April 1985 and was officially diagnosed in 1986. By June 1990, some 14,000 cases were confirmed (out of an estimated population of 10 million cattle) in Great Britain. Since 1986, more than 185,000 cases of BSE have been confirmed in Great Britain. The epidemic peaked in 1992-93 at almost 1,000 new cases per week. Control measures have since reduced incidence of the disease, and currently fewer than 100 new cases are reported per week. See similar questions...

Fistula occurs when emergency obstetric care is not available to women who develop complications during childbirth. This is why women living in remote rural areas with little access to medical care are at risk. Before the medical advances of the 20th Century, fistula was quite common in Europe and the United States. Today, fistula is almost unheard of in high-income countries, or in countries where obstetric care is widely available. See similar questions...

In 2007, there are approximately 2 million people with limb loss in the United States (excluding fingers and toes). There are more than 185,000 new amputations preformed each year in this country. The prevalence rate in 1996 was 4.9 per 1,000 persons. The incidence rate was 46.2 per 100,000 persons with dysvascular disease, 5.86 per 100,000 persons secondary to trauma, 0.35 per 100,000 secondary to malignancy of a bone or joint. The birth prevalence of congenital limb deficiency in 1996 was 25. See similar questions...

Sound enters your ears in sound waves. These sound waves hit your eardrum and are converted to pressure changes in the inner ear. This causes special nerves to impulse, which are read by your brain as sound. When these nerves are exposed to a surplus of pressure changes, they may become damaged. You may have noticed that if you listen to loud music for several hours that your ears "ring." This is a Temporary Threshold Shift, or a temporary reduction in hearing. See similar questions...

Conception, or fertilization, occurs when the male's sperm meets with the woman's ovum (egg) and successfully penetrates it. This typically takes place in one of the woman's fallopian tubes and the fertilized egg then travels to the uterus and implants in the uterine lining. For pregnancy to take place, fertilization of the egg must be followed by a successful implantation. Only one egg is released for fertilization in each menstrual cycle. See similar questions...

The theory about this that makes most sense is called Trophotropism. The easiest way to explain this is to make a comparison to a plant. A plant will lean towards the sun to get the light it needs to survive. Since the lower segment of the uterus is not as nourishing as the upper segment, the placenta will grow to reach more nourishing tissue. The placental mass will erode away from the cervix, but the vessels can't. This also explains the velamentous insertion of the cord. See similar questions...

Your body has a natural defense system called the immune system that protects you from infection and disease. The immune system defends your body by producing "killer" cells that destroy foreign substances (such as viruses and bacteria). Since the donor organ doesn't match your old organ exactly, your body tries to destroy the transplanted organ. Essentially, a transplant creates a new disease called rejection. Rejection is nature's way of protecting your body. See similar questions...

Almost everyone is exposed to low levels of asbestos in their lives. It exists in the air at extremely low concentration; levels are highest in urban and industrial areas. These fibers are released into the air during demolition work, remodeling, or other activity in which the material containing asbestos is disturbed. Asbestos may occur in tap water where such water passes through cement pipes containing the substance. See similar questions...

Preeclampsia can appear at any time during the pregnancy, delivery and up to six weeks post-partum, though it most frequently occurs in the final trimester and resolves within 48 hours of delivery. Preeclampsia can develop gradually, or come on quite suddenly, even flaring up in a matter of hours, though the signs and symptoms may have been present for months undetected or unnoticed. See similar questions...

Child abuse occur in the place where it is least expected; home, and from adults entrusted for the care of children; parents or guardians. In many cases, if the abuser is not a parent/guardian, s/he is certainly known to the child, especially in the case of sexual abuse. See similar questions...

Piles (hemorrhoids) are common in both men and women. By the age of 50 half of the people develop hemorrhoids to some extent. Although bleeding may occur in some cases, it does not cause any serious problem. But a person suffering from long standing bleeding piles (hemorrhoids) can have anemia. Women usually tend to develop piles (hemorrhoids) during pregnancy because of the pressure of the fetus and hormonal changes, but it is usually a temporary problem and disappears after childbirth. See similar questions...

In most instances, a corporation is administratively dissolved because it failed to file its annual registration with the Secretary of State. This often occurs because the corporation is "out of business." But it is also very common that the corporation moved its offices, the Secretary of State was not notified of the change, and the corporation's annual registration form was not forwarded to the new address. Many times the annual registration is not filed simply due to an oversight. See similar questions...

El Niño and La Niña occur on average every 3 to 5 years. However, in the historical record the interval between events has varied from 2 to 7 years. According to the National Centers for Environmental Prediction, this century's previous La Niñas began in 1903, 1906, 1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 1988, and 1995. These events typically continued into the following spring. See similar questions...

Most freshwater snails can self-fertilize for two or three generations without difficulty in genetic factors. Pulmonate snails will use cross-fertilized sperm in reproduction in preference to sperm from the same individual. This ability of self-fertilization is a very significant feature, because it enables the snail to establish a new colony by means of a single individual. This is only possible if the individual can hold out for a fairly long time at its new location. See similar questions...

Head lice have no problem finding their way from child to child. This is so readily enabled because children offer such a smaller personal zone than adults. In other words, kids tend to get close and contact one another during moments of play. As a result, a child that is infested creates an easy path for the lice on their head to pass across. See similar questions...

Melanoma can occur on any part of the body. Primary tumors are most common on the trunk in men and on the lower extremity (thighs, legs, feet) in women. Studies have concluded that incisional biopsies of melanoma do not cause the tumor to spread locally or metastasize. See similar questions...