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The body changes in amazing and complex ways during the course of pregnancy. During the first week of pregnancy, most of the changes going on within the body depend on whether a woman is counting from her last menstrual period or from conception. If a woman is counting from the first day of her last menstrual period, her body usually sheds its uterine lining in the form of a menstrual period during this week. If a woman counts the first week from conception, however, the cells that will become a baby are usually undergoing rapid changes within her reproductive system.
Figuring out what’s happening to the body during the first week of pregnancy can be complicated by the fact that doctors count pregnancy weeks from the first day of a woman’s last menstrual period. This is before a child is actually conceived, however, which means a woman is not actually pregnant. If a woman uses this 40-week method for considering the body changes of pregnancy, the body is shedding its uterine lining at this stage. A woman typically has menstrual bleeding at this time, and some women may experience pelvic cramping, mood swings, and an increase in acne as well. About 14 days after the beginning of this week, a woman’s ovary will usually release an egg that can be fertilized by her partner’s sperm.
If a woman considers the first week following conception rather than the first week of pregnancy as recognized in the medical world, she’ll learn that a complex series of activities are happening inside the body at this time. Once a sperm fertilizes an egg, a complicated process of cell multiplication and growth begins. This multiplication and growth involve the cells that will eventually become the embryo and the placenta. The cluster of cells does not look like a baby or even resemble a human being at this point in development.
In most cases, a woman won’t notice any changes in her body during this first week of pregnancy. At this point, a woman usually hasn’t even missed a period yet. Some women do say they notice symptoms as early as the first week after conception. It is possible, however, that they notice what seems like symptoms only because they are hoping to become pregnant or worried about the prospect of an unplanned pregnancy. If a woman does experience real symptoms during this time, such as nausea, fatigue, or breast tenderness, they might be caused by hormonal changes with the body.
@ElizaBennett - I never experienced implantation bleeding myself, but I think it's one reason why some women don't find out they're pregnant until they're kind of far along. If they have had light, irregular periods before, they could easily mistake implantation bleeding for a light period. But for most women, a period will mean they are definitely not pregnant, especially if there is bright red bleeding (as opposed to pink or brown spotting).
It's so weird to me that doctors count pregnancy from the last menstrual period, so that by the time you conceive, you are already about two weeks pregnant! But keeping track of ovulation can lead to a much more accurate due date. Most women will deliver their
babies about 266 days after ovulation, give or take a week on either side.
But ovulation doesn't always occur on day 14 of the menstrual cycle! It might be early or very late. If a woman ovulates late, say on day 25, her doctor might think her baby is a week "late" when really baby is not done growing yet! (Don't know when you conceived? Ask for an early ultrasound to verify the age of your baby. The earlier it's done, the more accurate.)
Something to realize about symptoms during the first week of pregnancy, or at least the first week after conception, is that they will depend in part on when implantation occurs.
If you think about it, your body can't really "tell" that fertilization has occurred. The egg continues on its way. When things start to change for the woman's body is when the fertilized egg implants in the lining of the uterus, which can happen anywhere from 6-12 days after fertilization (on average).
One early sign that a few women notice is implantation bleeding, which is really just spotting. But whether or not a woman experiences bleeding, she can start to experience other early pregnancy symptoms as soon as implantation occurs. For most women, though, a missed period will be the first definitive sign.