Even if you don't feel a thing (yet!), you're experiencing the amazing transformation of your egg into an embryo. Though the terms used to explain your body's changes during the first couple of weeks of pregnancy may sound clinical, they are a necessary part of describing the complex process of a baby's growth.
During week 1, your body sheds its uterine lining (that's when you get your period) and prepares to make a new one that's a hospitable nest for a fertilized egg. Meanwhile, about a thousand of your eggs make their way down the road to maturity. Only about 20 of those eggs will ripen inside fluid-filled sacs known as follicles, and then only one of those follicles will develop, ovulate, and rupture, allowing the egg (ovum) to start its trip down your fallopian tube during the 2nd week of pregnancy. (If two or more rupture, you may have twins, triplets, or more.)
Of course there can be no baby without a fertilized egg -- and that requires your partner's sperm. Typically, of the millions of sperm ejaculated into your vagina, only a few hundred will make it into your fallopian tube. Then the race is on, as the sperm swim upward toward the descending egg. Sperm can live inside a woman's body for as long as six days. Once an egg is released from the ovary, it must be fertilized within 12 to 24 hours. If the egg is not fertilized, it will simply pass through your body with menstrual flow.
How the sperm attaches to the egg is a marvel of biological engineering. Because your eggs have tough, protective shells, each sperm has a sticky structure at one end that allows it to attach to the egg's shell and tunnel right through it, taking along a tidy package of genetic material. Once one sperm penetrates the egg's lining, your egg is fertilized.
Conception typically takes place between days 14 and 17 of a regular 28-day menstrual cycle. It can take up to three days for the sperm to meet up with your egg in the fallopian tube, so the date of intercourse isn't always the same day your baby was conceived. (Remember those cautionary tales in high school health class?)
Your fertilized egg, which is now called an embryo, continues moving down the fallopian tube, growing larger as cells divide. If the egg implants in the fallopian tube, you will have an ectopic pregnancy but this is a rare occurrence. More commonly, the embryo cruises down the fallopian tube just fine, growing into a solid ball of about 200 cells by the time it enters your uterus about four days after conception. The embryo usually bounces around in the uterus for a few days until it finds a suitable spot to implant in the lining.
Don't be nervous if you experience a little spotting this week. The embryo burrows into your uterine lining as part of the implantation process, and this sometimes causes a small amount of bleeding. That's normal. Your baby's cells continue to multiply, and the placental tissue grows, inhabiting the lining of the uterus and establishing contact with your circulation, which will supply more blood to your growing baby. The placenta also produces estrogen, progesterone, and hCG (human chorionic gonadotropin), which is often referred to as the "pregnancy hormone."
Although your baby is still a microscopic cluster of cells at this point -- you can't even see it on an ultrasound -- three different essential cell layers are already beginning to develop. Their names are probably familiar from biology class: the ectoderm, which will become your baby's nervous system, hair, and skin; the endoderm, which will form your baby's gastrointestinal tract, liver, pancreas, and thyroid; and the mesoderm, which eventually develops into your baby's skeleton, connective tissue, blood system, urogenital system, and muscles.
Originally published in You & Your Baby: Pregnancy.
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