Even if you don't feel a thing (yet!), you're experiencing the amazing transformation of your egg into an embryo and may still have some 1st week pregnancy symptoms. 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 and determining the first signs of life.
During your first week of pregnancy, 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.)
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?)
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.
At the beginning, calculating your baby-to-be's due date is no easy task! To figure out your baby's expected arrival day, your health care provider will ask you when your last menstrual period (LMP) began. For most women, the average menstrual cycle lasts 28 days long, but normal range falls anywhere between 21 to 35 days in adult women (and from 21 to 45 days for young teens.
While most women know the date when their last period began, most wouldn't know the day they ovulated. For that reason, health care providers use the LMP as a starting point for guesstimating the day of your baby's arrival. The most accurate way to figure out her arrival date is with an ultrasound, but not all health care providers request an ultrasound during the first trimester of pregnancy.
Ovulation: The point at which the hormones in a woman's body signal the ovaries to release an egg into the Fallopian tube. This egg can then either be fertilized by a sperm or continue to the uterus, where it is released from the body during menstruation.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).