Because the date of ovulation is often difficult to pinpoint, medical experts usually use the first day of your last period as the official beginning of the 40 weeks of pregnancy, and as the starting point in calculating your baby's gestational age.
Therefore, the extraordinary process by which your baby comes to life actually begins with an utterly ordinary event--your monthly period. During your last period before conception, the groundwork for pregnancy is already being laid. The previous month's uterine lining, called the endometrium, is shed, along with some blood, resulting in menstrual flow. Meanwhile, follicle stimulating hormone (FSH) kicks into gear, triggering the development of several ovarian follicles. Located deep in the ovary, these yellow bulbs each contain a single immature egg. As the bulbs grow, the eggs begin to mature. The follicles also signal the ovary to produce estrogen, a key hormone in bringing about ovulation.
Inside their comfy ovarian follicles, the eggs, each of which contains half the genes needed to create a human being, are maturing, ballooning to over three times their original size. Hormones continue to do their work: rising levels of estrogen trigger a surge of luteinizing hormone, or LH. This rush of LH causes the largest of the developing follicles to rupture, spewing its egg into one of two fallopian tubes. Ovulation, as this big event is known, usually happens around day 14 to 18. If you're trying to conceive, it helps to have a good idea of when your ovulation will occur -- the prime time for intercourse is the two days before and the day of ovulation.
So what happens when you and your partner are busy making babies? On average, about 350 million sperm spew forth from the man's body during an ejaculation. But during intercourse, only about 1 percent, or 3 million, will make their way through the uterus to the fallopian tube, where the egg is waiting. The sperm may need about ten hours to make this journey, struggling through thin channels in the cervical mucus, propelled by lashings of their tiny tails. Sperm have been known to survive anywhere from two to five days inside a woman's body, but the egg is viable only for approximately 12 to 24 hours after ovulation. Clearly, in the conception game, timing is everything. The window of opportunity is narrow, and that's why even the healthiest couples have about a 15 to 20 percent chance of conceiving each month they try.
You and your partner may have finished your baby-making endeavors by now, but inside your body, the action has just begun. Of the original 300 million-plus sperm, just 250 have found their way to the right spot: the mature egg. The fertilization of the egg begins when one lucky sperm penetrates the egg's tough outer membrane. It takes about twenty minutes for the sperm to completely pierce this "shell." Within the following eleven hours, the sperm's nucleus merges with the nucleus of the egg. With this momentous melding of genetic goodies, fertilization is complete. The egg becomes a zygote, the first step in its journey toward babyhood.
The zygote then begins to drift down the fallopian tube toward the uterus. Along the way, its single cell splits into two cells, then four, and so on, dividing about every 12 hours. About four days after fertilization, when it has about sixteen cells and looks like a tiny raspberry, the cluster, now called a morula, jostles its way along the narrowest part of the fallopian tube and finally enters the uterus, which is primed to receive it. A hormonal brew of estrogen and progesterone (the latter produced by the ruptured ovarian follicle) has triggered the growth of a nutrient-rich bed of blood vessels, the perfect home for the yet-to-be-hatched embryo.
The fertilized egg has seen a whole lot of growth in the days since it was first penetrated by that single adventurous sperm. And this week, safe inside the uterus, it's changing yet again. A cavity forms its center, and two groups of cells form on its sides: one group will become the embryo, and the other will become the placenta. Meanwhile, the amniotic fluid is collecting, and the yolk sac, which will nourish the embryo in its earliest days, is forming. At this stage, the morula becomes a blastocyst.
About six or seven days after fertilization, the blastocyst "hatches," tumbling out of its surrounding membrane so its growing mass of cells can more easily bond with the uterus. Using "fingers," called chorionic villi, the blastocyst anchors itself in the lush uterine lining (the ideal implantation site is the back wall of the uterus, toward the mother's spine). This watershed event marks the real start of incubation, as this tiny speck of living matter--now called an embryo--and its mother become linked together, sharing hormones and other essential fluids.
Because hormones from the embryo are now flowing through your bloodstream, a blood test taken at now would very likely detect the pregnancy, even though it's still a week to ten days before your period has been missed.
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