Читать книгу Dad's Guide to Pregnancy For Dummies - Sharon Perkins - Страница 46

Women produce one egg a month, most of the time, and men produce millions of sperm. Why the huge disparity? Because one sperm isn’t enough to get the job done — it needs lots of friends to help. Although only one victorious sperm makes it into the egg, breaking down the coating that surrounds the egg takes many sperm. And although eggs get to drift downward from the ovary to the fallopian tube, sperm have to swim upstream. Needless to say, some fall by the wayside, either because they tire or because they take a wrong turn somewhere.

Sperm are also produced in large quantities because many are abnormal, having two tails, no tails, round tails, small heads, large heads, or abnormally shaped heads. Abnormal tails make navigation difficult, and abnormal heads often indicate chromosomal abnormalities.

Only 50 to 60 percent of sperm need good motility, or movement, for a sperm sample to be considered normal, so lots of sperm don’t make the grade, creating a need for higher numbers.

Even worse, the embryo may implant in the fallopian tube, a situation known as an ectopic pregnancy. The tube has no room for a developing fetus, so an ectopic pregnancy is doomed from the start and can cause serious, life-threatening bleeding if the tube ruptures. Ectopic pregnancies occur in around 2 percent of all pregnancies.

Even after the embryo reaches the uterus, it’s not always clear sailing. The uterine lining has to be just right for implantation. Estrogen thickens the lining before ovulation, and progesterone released from the corpus luteum (the leftover shell of the follicle that contained the egg) prepares the lining after ovulation. If either of these hormone levels is low, the lining may not be able to support a pregnancy. Note: Your partner’s medical practitioner can assess the uterine lining by ultrasound and prescribe extra progesterone if needed to achieve and maintain pregnancy.

After the embryo reaches the uterus and implants, the implanted embryo begins to produce human chorionic gonadotropin, or hCG, the hormone that pregnancy tests measure. hCG levels aren’t detectable until the embryo implants, or around the time of the first missed period.