How Sperm Meets Egg Parents
Your due date is calculated from the firstday of your last menstrual period. Conception occursabout two weeks from this day, and that's when you're truly consideredto be pregnant. During a menstrual cycle, one ofyour two ovaries matures an egg within a follicle. Hormonalsurges controlled by your brain then cause one and occasionallytwo follicles to release an egg. If both ovaries release anegg, you may have nonidentical twins, also known asfraternal twins. Long,
fingerlike projections sweep the egg intothe fallopian tube. The egg travels down the fallopian tube, pushedby tiny hairs and awaits the arrival or sperm. About a teaspoonworth of semen enters the vagina containing roughly300 million sperm, less than 100,000 will pass into the cervixto begin their 6inch journey to the egg. A long, whipliketail propels each sperm toward the waiting egg.Most sperm will lose their way. Only about 200 sperms successfullyreach the
egg. Fertilization occurs when one sperm penetratesthe egg, combining their genetic material. The sperm and egg,each contain 23 chromosomes holding all of the informationthat determines your child's sex and contributes to the child's geneticmake up, including physical appearance, intelligence, and personality.It's a boy or a girl; you just don't know which yet. Within24 hours of fertilization, the egg, now called a zygote,divides into two cells. The number of cells doubles aboutevery twelve
hours as the zygote continues its trek tothe uterus. During this early stage, if the cells splitinto twoseparate groups, instead of remaining attached, you'llbe doubly blast with identical twins. About three days after fertilization,the zygote has become a ball of 32 cells resembling a miniatureraspberry. A few days later, it reaches the uterus asa ball of roughly 500 cells surrounding a fluidfield cavity. On day 7, the ball of cells burrows into uterinewall during a process called implantation.
Many women have implantationbleeding or spotting at this time which occurs about 21days after a period, before a pregnancy test could evenbe positive. The cells on the outside of the ballcreate the yolk sac and the placenta, which nourishes yourdeveloping baby. By 38 weeks, the fertilized egg has growninto an infant with more than 2 trillion cells.