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What Happens During the First Week of Pregnancy?

written by: Dr. Sloan, MD. • edited by: BStone • updated: 10/31/2010

What happens the first week of pregnancy? Most women are not even aware that they are pregnant. Possibly, the first signs that they might notice could be a missed period, headaches, unusual tiredness and mood swings which they could easily have dismissed as symptoms of stress.

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    What Happens the First Week of Pregnancy?

    It may sound strange but true that by the time a woman notices a positive result in a urine pregnancy test, she could have been pregnant for about 4 weeks. Though it may be difficult to know the exact date of when the fertilization took place, it is not impossible to estimate the expected date of delivery. Counting 40 weeks ahead from the first day of the last menstrual period will reveal the expected date of delivery. Keeping that in mind, the fertilization or conception only happens 2 weeks after the last menstrual period. Thus, part of the period of the last cycle is considered as the beginning part or first week of the pregnancy.

    Usually the ovum is fertilized in the ampulla portion of the fallopian tube. As a result, the zygote is formed with complete genetic makeup. Depending on the combination of sex chromosomes (XX-Female or XY-Male), the sex of the foetus is determined. However, if the zygote divides to form two separate embryos, a twin pregnancy can be expected.

    Soon after fertilization, a series of cells divisions take place as they progressively move towards the uterus. Such migration from the site of fertilization to the uterus may take up to 2 weeks after fertilization. The embryo, placenta and the membranes that surround the growing embryo are all specialized tissues generated from these cells. About 10 to 14 days after fertilization, the blastocyst adheres itself to the endometrium wall of the uterus.

    Once the implantation takes place, the placenta provides essential nourishment to the growing embryo via the blood vessels from the mother. The hCG hormone used to detect pregnancy in urine pregnancy tests is also produced by the placenta. However, if the implantation does not occur, the tissues will be washed out with the next menstrual cycle.

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    Symptoms of First Week of Pregnancy

    Most of the women are not even aware if they are pregnant during that first week of pregnancy. One of the earliest symptoms of pregnancy could be a missed period coupled with uncommon fatigue, headaches and mood swings caused by hormonal changes in the body. Most of the other signs of pregnancy such as tenderness of breasts and morning sickness would only start in the second week of pregnancy.

    Implantation spotting which is characterized by scanty brownish discharged, unlike the normal menstrual bleeding may be present after the blastocyst attaches itself to the uterine wall.

    The most sensitive pregnancy tests can potentially detect human chorionic gonadotropin or hCG levels at as low as 20 mIU. This level of hCG is present in the urine of pregnant women at about 7 to 10 days past ovulation.

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    References

    1) Foetal Development Month by Month, reviewed by Dr Louise Chang MD

    2) "Current knowledge of the aetiology of human tubal ectopic pregnancy". Shaw JL, Dey SK, Critchley HO, Horne AW (January 2010).

    3) Fetal development: First Trimester Week 1 by Mayo Clinic Staff

    4) The Obstetrics Textbook by Mudaliar 9th edition