Human Reproduction and Natural Conception
The testicles in the male and the ovaries in the female are the body organs which produce both the sex hormones and the reproductive cells (AKA gametes): sperm cells and eggs, respectively. The union of a single sperm cell and an egg, in a process called fertilization, results in the formation a new cell (AKA zygote), which contains the genetic information (in the chromosomes) of both its “parent” cells. The zygote then develops into an embryo, which is the very early formation of a new being.
Sperm cells are constantly being produced in the testicles of the adult male. This process begins at puberty and continues throughout the remainder of the man's life, with minor declines in old age. The formation of a mature sperm takes about 70-90 days, during which it becomes a cell that not only contains the male genetic information, but also is capable of propelling itself and travel through the female tract (cervix and uterus) to reach and fertilize the egg.
Of the millions of sperm deposited in the vagina at the time of intercourse and ejaculation, only a few hundred survive and make it into the fallopian tube – where fertilization normally occurs. The ejaculate consists not only of the sperm but also of several cc's of fluid from the prostate, which nourishes and protects the sperm. Since only the sperm enter the cervix (and a very small percentage of it), it is normal for the majority of the ejaculate to leak out of the vagina following intercourse. Sperm which enters the cervix into the uterus and tubes may survive for several days, during which it is able to fertilize the mature egg.
As opposed to the ongoing production of the sperm, new eggs are not formed within the ovaries. All of the eggs a woman will have, usually about two million, are present when she is born, and the number continues to decrease until, at the time of menopause, when the egg supply is depleted. This explains why fertility declines abruptly in older women, while older men typically retain their fertility potential into older age.
After puberty, during each menstrual cycle several eggs begin to develop over a period of 2-3 weeks, but usually just one of the eggs will reach maturity, while the rest degenerate and are lost forever. This egg is contained in a fluid filled compartment (follicle) which is slowly growing until the egg is mature and it then is released from the ovary, surrounded by its protective cells, a process called ovulation. The egg will be capable of being fertilized for only the next 12 to 24 hours at most. If not fertilized within that time, it is simply reabsorbed or lost from the body.
The fallopian tube is the trumpet-like structure that sits between the uterus and ovary. After ovulation the egg does not just fall into the tube, it is picked up by finger-like projections on the end of the tube near the ovary. It is then transported inside of the tube toward the uterus into the portion of the tube known as the ampulla. It is here that the sperm and egg meet and that fertilization will occur.
It doesn't take just one sperm to fertilize an egg. There are many protective cells surrounding the egg, and many sperm are lost while actively removing these cells in order to gain access to the egg. Only after these cells have been removed and a path cleared can a single sperm penetrate the egg. Once a sperm penetrates the egg, the protective layer around the egg immediately undergoes changes that prevent any further sperm from entering the egg.
After fertilization, the zygote, or early embryo, remains in the tube for another three or four days. While in the tube, continued development occurs. When the embryo is transported t into the uterus, it is usually about 20 to 40 cells in size. The embryo "floats" in the uterus for an additional couple of days before attaching and penetrating into the wall of the uterus, a process known as implantation.
The menstrual cycle is the time from the start of one “period” (AKA menses or menstrual bleeding) to the start of the next. The most common menstrual cycle is 28-30 days long and is regular (the menses appear at regular intervals, which means that the majority of cycles have the same length). However, cycles of shorter or longer length or if they are irregular, may be or may not be normal. A “normal” cycle is one which is associated with ovulation.
We define Day 1 of a cycle as the first day of normal menstrual flow. In a typical 28-days cycle, ovulation occurs on Day 14 and the part of the cycle until ovulation, during which the egg matures, is called the "follicular phase" (because the egg develops in a fluid-filled small sac/cyst within the ovary, called the follicle). The follicle can be seen by transvaginal Ultrasound when it is 8-10 mm in diameter early in the follicular phase and it reaches 18-20 mm close to the time of ovulation. As the follicle (and the egg within it) develops, it produces rising levels of hormones, the most important of which is estradiol (the primary form of estrogen). Estradiol is responsible for stimulating changes in the cervix and the lining (mucosa) of the uterus (AKA , Endometrium), which are favorable for fertilization (of the egg) and implantation (of the embryo).
After ovulation, the now empty follicle will change its color to yellow (this is why it is called the “yellow body” or Corpus Luteum, in Latin) and will start to produce increasing amounts of another hormone, Progesterone, which makes the uterine mucosa thicker, spongy and more receptive to the development of the embryo and its nourishing organ, the placenta. The second half of the menstrual cycle, after ovulation, is also called the “Luteal phase”.
If no conception occurs in any given cycle (a zygote does not form or no implantation has occurred), Progesterone production by the Corpus Lutem (C-L) will decrease abruptly, and the menstrual bleeding will appear exactly 13-14 days after ovulation. This is because the life span of the Corpus Luteum is very exact. The menstrual bleeding is actually the shedding or sloughing out of the thickened Endometrium after it has been “primed” by Estrogen and then Progesterone. Thus, a new cycle starts with a very thin Endometrium, which under the influence of Estardiol and Progesterone will increase in thickness and nourishment.
If conception does occur and the embryo implants in the thick Endometrium, a unique “pregnancy hormone” (AKA human Chorionic Gonadotropin, or hCG) is produced by the primitive placenta of the embryo. Very low levels of this hormone may be detected in the blood few days before the anticipated menstrual period, or 10-12 days after ovulation. Early pregnancy tests are based on the detection of hCG in blood. This hormone is responsible for the maintenance of the Corpus Luteum beyond its regular lifespan and more importantly, the increasing production of Progesterone, which is necessary for pregnancy maintenance and development. Only 2-3 weeks later (at 6-7 weeks of gestational age, defined from the 1st day of the last period) the placenta assumes most of the hormone production of the C-L and pregnancy maintenance is no longer dependent on the function of the C-L.
There are only a few days at most in any given menstrual cycle during which conception can occur. While sperm may survive for several days in the reproductive tract of the female, the egg is only healthy and capable of being fertilized for 24 hours at most. The best chance of conception comes when a couple has intercourse one to two days before ovulation.
If a woman has a regular cycle length of 28 days, she will ovulate at mid-cycle (Day 14), or 14 days after first day of her period. If the cycle is longer, say 34 days and regular, ovulation occurs around Day 20 of the cycle, not mid-cycle. So if the menstrual cycle is regular ovulation can be predicted by using this calculation and so you can determine when the best chance of conception will be.
Some women know when they are ovulating from changes in their body and the way they feel. Some typical indicators are breast soreness, heavier and more opaque vaginal discharge, tightness in the abdomen. But many others have no noticeable symptoms.
Another approach would be to have regular intercourse 2 or 3 times a week no matter where a woman is in her cycle or in the week when she is most likely to ovulate (Day 10-16 of a 28-30 day cycle).
Finally, ovulation can be predicted by monitoring follicle growth with transvaginal Ultrasound or even better a home urine ovulation detector, which picks up the presence of “Luteinizing hormone” (LH). Typically LH can be detected in the urine 24-36 hours before ovulation, a good timing to have sex!.
Pregnancy is a complicated process and its success (Livebirth) depends on many factors:
• The production of normal sperm by the man and normal eggs by the woman
• Unblocked fallopian tubes that allow passage of the sperm to reach the egg and of the zygote to reach the uterus
• The sperm’s ability to fertilize the egg and form the zygote
• The development of a genetically healthy embryo from the zygote
• The ability of the embryo to implant in the uterus
• The development of healthy normal fetus (after the formation of all the body organs, approximately 10 weeks after conception, it is called a fetus)
• Normal growth and maturation of all body systems to allow survival outside the uterus
Repeatedly encountering difficulty at any of these steps can lead to infertility and reproductive failure (poor pregnancy outcomes). The reality is that human reproduction is a fairly inefficient process. For the average fertile (healthy) couple, having intercourse around the time of ovulation, the chance of fertilization is about 80%, but by the time of the expected menstrual period, roughly half of the early embryos have already failed to develop or implant. Some embryos will implant but are fundamentally abnormal and are unable to survive. In fact, the menstrual period might not even be delayed and the couple does not realize that an early pregnancy has been lost. If a menstrual period is missed (that is “clinical pregnancy”), a quarter or more of the remaining embryos can still fail later, resulting in abortion or fetal death. It can be calculated that in any given natural cycle, the normal, fertile couple has only a 20% chance (1 in 5) that intercourse at the right timing (just before ovulation) will result in a livebirth! (i.e., a baby that can survive).
Human reproduction is a tremendously age-sensitive event and for reasons which we have described before, the age of the woman is the most important. Peak fertility is achieved in the early twenties and it decreases slowly with advancing age. The decrease is more significant after 36 years of age and even more sharply after 39-40 years of age (see graph). While the chances to conceive naturally in any given month are 25-30% when the woman is in her twenties, it is 15-20% in the early thirties, 10-15% in the late thirties, 5-8% in the early forties and only 1% or less at the age of 45. After this age, natural conceptions are very rare. However, it should be noted that these rates are monthly conception rates and every month we are facing this chance again and again. This is why most healthy couples (about 90%) will conceive within 1 year and the cumulative chance to get pregnant is around 90% after 1 year of “unprotected intercourse” (i.e, without contraception).
Many couples worry that it may be taking them too long to conceive. This is a natural concern, but studies show that psychological stress and anxiety may further decrease our chances to conceive. In this modern world, we have become increasingly used to controlling anything and everything, and to making things happen when we want. But we can't make ourselves be pregnant when we want. Nature doesn't work that way. Nature requires patience, and some couples may need more patience than others. Having this perspective and assuming self-control and peace of mind are better strategies, which will result in natural pregnancies and avoid unnecessary interventions.
Finally the real question is: "How long should a couple be patient before they begin to seek some help and evaluation?" The answer is that a young couple with no previous pregnancies probably should wait 1 year and even longer, while if they had prior pregnancy(ies) or when the woman is older than 35, a six months “natural” trial period is more appropriate. These are, however, only definitions and guidelines. They do not mean that any couple must wait a mandatory year before they begin to seek some evaluation and help. When a couple becomes concerned about their ability to conceive, they should schedule some time with a physician and talk it over. Depending on the circumstances, it may be that some simple reassurance is all that is warranted. While it may not be appropriate to become overly concerned and perform a lot of expensive and extensive testing, some simple evaluation may go a long way toward reassurance.
Sperm cells are constantly being produced in the testicles of the adult male. This process begins at puberty and continues throughout the remainder of the man's life, with minor declines in old age. The formation of a mature sperm takes about 70-90 days, during which it becomes a cell that not only contains the male genetic information, but also is capable of propelling itself and travel through the female tract (cervix and uterus) to reach and fertilize the egg.
Of the millions of sperm deposited in the vagina at the time of intercourse and ejaculation, only a few hundred survive and make it into the fallopian tube – where fertilization normally occurs. The ejaculate consists not only of the sperm but also of several cc's of fluid from the prostate, which nourishes and protects the sperm. Since only the sperm enter the cervix (and a very small percentage of it), it is normal for the majority of the ejaculate to leak out of the vagina following intercourse. Sperm which enters the cervix into the uterus and tubes may survive for several days, during which it is able to fertilize the mature egg.
As opposed to the ongoing production of the sperm, new eggs are not formed within the ovaries. All of the eggs a woman will have, usually about two million, are present when she is born, and the number continues to decrease until, at the time of menopause, when the egg supply is depleted. This explains why fertility declines abruptly in older women, while older men typically retain their fertility potential into older age.
After puberty, during each menstrual cycle several eggs begin to develop over a period of 2-3 weeks, but usually just one of the eggs will reach maturity, while the rest degenerate and are lost forever. This egg is contained in a fluid filled compartment (follicle) which is slowly growing until the egg is mature and it then is released from the ovary, surrounded by its protective cells, a process called ovulation. The egg will be capable of being fertilized for only the next 12 to 24 hours at most. If not fertilized within that time, it is simply reabsorbed or lost from the body.
The fallopian tube is the trumpet-like structure that sits between the uterus and ovary. After ovulation the egg does not just fall into the tube, it is picked up by finger-like projections on the end of the tube near the ovary. It is then transported inside of the tube toward the uterus into the portion of the tube known as the ampulla. It is here that the sperm and egg meet and that fertilization will occur.
It doesn't take just one sperm to fertilize an egg. There are many protective cells surrounding the egg, and many sperm are lost while actively removing these cells in order to gain access to the egg. Only after these cells have been removed and a path cleared can a single sperm penetrate the egg. Once a sperm penetrates the egg, the protective layer around the egg immediately undergoes changes that prevent any further sperm from entering the egg.
After fertilization, the zygote, or early embryo, remains in the tube for another three or four days. While in the tube, continued development occurs. When the embryo is transported t into the uterus, it is usually about 20 to 40 cells in size. The embryo "floats" in the uterus for an additional couple of days before attaching and penetrating into the wall of the uterus, a process known as implantation.
The menstrual cycle is the time from the start of one “period” (AKA menses or menstrual bleeding) to the start of the next. The most common menstrual cycle is 28-30 days long and is regular (the menses appear at regular intervals, which means that the majority of cycles have the same length). However, cycles of shorter or longer length or if they are irregular, may be or may not be normal. A “normal” cycle is one which is associated with ovulation.
We define Day 1 of a cycle as the first day of normal menstrual flow. In a typical 28-days cycle, ovulation occurs on Day 14 and the part of the cycle until ovulation, during which the egg matures, is called the "follicular phase" (because the egg develops in a fluid-filled small sac/cyst within the ovary, called the follicle). The follicle can be seen by transvaginal Ultrasound when it is 8-10 mm in diameter early in the follicular phase and it reaches 18-20 mm close to the time of ovulation. As the follicle (and the egg within it) develops, it produces rising levels of hormones, the most important of which is estradiol (the primary form of estrogen). Estradiol is responsible for stimulating changes in the cervix and the lining (mucosa) of the uterus (AKA , Endometrium), which are favorable for fertilization (of the egg) and implantation (of the embryo).
After ovulation, the now empty follicle will change its color to yellow (this is why it is called the “yellow body” or Corpus Luteum, in Latin) and will start to produce increasing amounts of another hormone, Progesterone, which makes the uterine mucosa thicker, spongy and more receptive to the development of the embryo and its nourishing organ, the placenta. The second half of the menstrual cycle, after ovulation, is also called the “Luteal phase”.
If no conception occurs in any given cycle (a zygote does not form or no implantation has occurred), Progesterone production by the Corpus Lutem (C-L) will decrease abruptly, and the menstrual bleeding will appear exactly 13-14 days after ovulation. This is because the life span of the Corpus Luteum is very exact. The menstrual bleeding is actually the shedding or sloughing out of the thickened Endometrium after it has been “primed” by Estrogen and then Progesterone. Thus, a new cycle starts with a very thin Endometrium, which under the influence of Estardiol and Progesterone will increase in thickness and nourishment.
If conception does occur and the embryo implants in the thick Endometrium, a unique “pregnancy hormone” (AKA human Chorionic Gonadotropin, or hCG) is produced by the primitive placenta of the embryo. Very low levels of this hormone may be detected in the blood few days before the anticipated menstrual period, or 10-12 days after ovulation. Early pregnancy tests are based on the detection of hCG in blood. This hormone is responsible for the maintenance of the Corpus Luteum beyond its regular lifespan and more importantly, the increasing production of Progesterone, which is necessary for pregnancy maintenance and development. Only 2-3 weeks later (at 6-7 weeks of gestational age, defined from the 1st day of the last period) the placenta assumes most of the hormone production of the C-L and pregnancy maintenance is no longer dependent on the function of the C-L.
There are only a few days at most in any given menstrual cycle during which conception can occur. While sperm may survive for several days in the reproductive tract of the female, the egg is only healthy and capable of being fertilized for 24 hours at most. The best chance of conception comes when a couple has intercourse one to two days before ovulation.
If a woman has a regular cycle length of 28 days, she will ovulate at mid-cycle (Day 14), or 14 days after first day of her period. If the cycle is longer, say 34 days and regular, ovulation occurs around Day 20 of the cycle, not mid-cycle. So if the menstrual cycle is regular ovulation can be predicted by using this calculation and so you can determine when the best chance of conception will be.
Some women know when they are ovulating from changes in their body and the way they feel. Some typical indicators are breast soreness, heavier and more opaque vaginal discharge, tightness in the abdomen. But many others have no noticeable symptoms.
Another approach would be to have regular intercourse 2 or 3 times a week no matter where a woman is in her cycle or in the week when she is most likely to ovulate (Day 10-16 of a 28-30 day cycle).
Finally, ovulation can be predicted by monitoring follicle growth with transvaginal Ultrasound or even better a home urine ovulation detector, which picks up the presence of “Luteinizing hormone” (LH). Typically LH can be detected in the urine 24-36 hours before ovulation, a good timing to have sex!.
Pregnancy is a complicated process and its success (Livebirth) depends on many factors:
• The production of normal sperm by the man and normal eggs by the woman
• Unblocked fallopian tubes that allow passage of the sperm to reach the egg and of the zygote to reach the uterus
• The sperm’s ability to fertilize the egg and form the zygote
• The development of a genetically healthy embryo from the zygote
• The ability of the embryo to implant in the uterus
• The development of healthy normal fetus (after the formation of all the body organs, approximately 10 weeks after conception, it is called a fetus)
• Normal growth and maturation of all body systems to allow survival outside the uterus
Repeatedly encountering difficulty at any of these steps can lead to infertility and reproductive failure (poor pregnancy outcomes). The reality is that human reproduction is a fairly inefficient process. For the average fertile (healthy) couple, having intercourse around the time of ovulation, the chance of fertilization is about 80%, but by the time of the expected menstrual period, roughly half of the early embryos have already failed to develop or implant. Some embryos will implant but are fundamentally abnormal and are unable to survive. In fact, the menstrual period might not even be delayed and the couple does not realize that an early pregnancy has been lost. If a menstrual period is missed (that is “clinical pregnancy”), a quarter or more of the remaining embryos can still fail later, resulting in abortion or fetal death. It can be calculated that in any given natural cycle, the normal, fertile couple has only a 20% chance (1 in 5) that intercourse at the right timing (just before ovulation) will result in a livebirth! (i.e., a baby that can survive).
Human reproduction is a tremendously age-sensitive event and for reasons which we have described before, the age of the woman is the most important. Peak fertility is achieved in the early twenties and it decreases slowly with advancing age. The decrease is more significant after 36 years of age and even more sharply after 39-40 years of age (see graph). While the chances to conceive naturally in any given month are 25-30% when the woman is in her twenties, it is 15-20% in the early thirties, 10-15% in the late thirties, 5-8% in the early forties and only 1% or less at the age of 45. After this age, natural conceptions are very rare. However, it should be noted that these rates are monthly conception rates and every month we are facing this chance again and again. This is why most healthy couples (about 90%) will conceive within 1 year and the cumulative chance to get pregnant is around 90% after 1 year of “unprotected intercourse” (i.e, without contraception).
Many couples worry that it may be taking them too long to conceive. This is a natural concern, but studies show that psychological stress and anxiety may further decrease our chances to conceive. In this modern world, we have become increasingly used to controlling anything and everything, and to making things happen when we want. But we can't make ourselves be pregnant when we want. Nature doesn't work that way. Nature requires patience, and some couples may need more patience than others. Having this perspective and assuming self-control and peace of mind are better strategies, which will result in natural pregnancies and avoid unnecessary interventions.
Finally the real question is: "How long should a couple be patient before they begin to seek some help and evaluation?" The answer is that a young couple with no previous pregnancies probably should wait 1 year and even longer, while if they had prior pregnancy(ies) or when the woman is older than 35, a six months “natural” trial period is more appropriate. These are, however, only definitions and guidelines. They do not mean that any couple must wait a mandatory year before they begin to seek some evaluation and help. When a couple becomes concerned about their ability to conceive, they should schedule some time with a physician and talk it over. Depending on the circumstances, it may be that some simple reassurance is all that is warranted. While it may not be appropriate to become overly concerned and perform a lot of expensive and extensive testing, some simple evaluation may go a long way toward reassurance.