| The first trimester: Month one | 329.000000000000 | The first trimester: Month one | The first trimester: Month one | T | English | Pregnancy | Adult (19+) | Body | Reproductive system | NA | Prenatal
Adult (19+) | NA | | 2009-09-11T04:00:00Z | | | | | | 8.70000000000000 | 56.7000000000000 | 861.000000000000 | | Flat Content | Health A-Z | <p>Learn about fetal development in the first month of pregnancy. Ovulation, fertilization, and miscarriage are discussed.</p> | <p>The Pregnancy & Babies resource centre counts the weeks of pregnancy starting with the date of the beginning of the woman's last menstrual period, which is estimated to be about two weeks before the date of conception. Therefore, the countdown of fetal development begins at week three after the beginning of the woman's last menstrual period. When counted this way, pregnancy will last a total of 40 weeks.</p> | | <h2>Key points</h2>
<ul><li>Pregnancy will last a total of 40 weeks, starting from the date of the beginning of the woman's last menstrual period.</li>
<li>Fertilization happens during the third week of pregnancy.</li>
<li>There is a 45% rate of miscarriage during the first two weeks after fertilization.</li></ul> | <figure><span class="asset-image-title">Fertilization</span>
<div class="asset-animation"> src="https://akhpub.aboutkidshealth.ca/Style%20Library/akh/swfanimations/swf.html?swffile=Fertilization_MED_ANI_EN.swf" </div></figure>
<h2>Ovulation and fertilization<br></h2><p>Once every month or so, a woman’s ovary releases an egg cell. This process, known as ovulation, usually occurs about 14 days after the woman’s last menstrual period. The egg cell is swept down the fallopian tube, which lies between the ovary and the uterus. The egg cell slows in the upper part of the fallopian tube. This is where the egg cell is usually fertilized by a man’s sperm cell.</p><p>During sex, the man's penis releases a thick, whitish fluid called semen into the woman’s vagina. Semen contains millions of sperm cells that deposit in and around the vagina. Several thousand of these sperm cells pass through the vagina into the uterus. From there, the sperm cells enter the fallopian tubes. The sperm cells surround the egg cell if it is present. When a sperm cell enters an egg cell, they unite and their chromosomes mingle together. This is called conception or fertilization.</p><h2>Week three of pregnancy<br></h2><p>Human development begins at fertilization. The normal fertilized egg cell contains 23 chromosomes from the mother and 23 from the father. Thus, there are normally 23 pairs of chromosomes in the fertilized egg. These include two sex chromosomes: XX for girls and XY for boys. At this point, the newly fertilized egg is called an embryo.</p><p>Within 24 hours after fertilization, the embryo divides into two cells. Soon it divides into four cells, then eight, and so on. During this series of cell divisions, the embryo continues to move along the fallopian tube toward the uterus.</p><p>For one to three days after fertilization, the embryo goes through more and more cell divisions. As this happens, the embryo changes shape and size. Its cells form into a tight ball. Three days after fertilization, there is a cluster of about 12 to 16 cells. At this point, the embryo enters the uterus.</p><p>Once in the uterus, the ball of fertilized cells floats around for approximately 24 hours. During this time, fluid-filled spaces begin to appear between some of the embryo’s cells. Soon the cells separate into two parts: a thin outer cell layer and an inner cell mass. The outer layer will eventually form the placenta and the inner cell mass will develop into the baby.</p><p>While it is floating in the uterine cavity, the embryo gets nourishment from secretions of glands in the uterus.</p><h2>Week four of pregnancy<br></h2>
<figure><span class="asset-image-title">Implantation</span><img src="https://assets.aboutkidshealth.ca/akhassets/Implantation_week4_MED_ILL_EN.jpg" alt="Embryo on the lining of the uterus with arrow indicating attachment" /> </figure>
<p>Six to 10 days after fertilization, the embryo attaches, or implants, itself into the lining of the uterus. During the next week or so, the embryo receives its nourishment and oxygen from the cells that make up the lining of the uterus.</p><p>The two layers of cells become more pronounced. A structure called the yolk sac starts to form. The yolk sac will provide nutrients to the embryo while the placenta is developing. Special networks begin to form between the embryo and the uterine wall, through which blood from the mother starts to flow.</p><p>During implantation, the embryo begins producing a hormone called human chorionic gonadotropin (hCG), which enters the mother’s blood. hCG maintains the lining of the uterus. Pregnancy tests measure the level of this hormone.</p><p>Implantation is complete by the end of this week. This is a busy time for the embryo! All major external and internal structures and organ systems begin to develop during the next few weeks.</p><h2>What concerns are there in these early weeks of pregnancy?</h2><p>The first few weeks of pregnancy are a delicate time when miscarriage is common. There is a 45% rate of miscarriage during the first two weeks after fertilization. Miscarriage is also referred to as spontaneous abortion. At this stage in pregnancy, miscarriage is usually caused by abnormalities within the embryo or problems with implantation. If you have a miscarriage during this time, chances are that you will not realize it.</p><p>Another concern during this time is a condition known as ectopic pregnancy. An ectopic pregnancy occurs when the embryo implants outside the uterus, usually in the fallopian tube. An ectopic pregnancy is often related to a problem with the transport of the embryo through the fallopian tube.</p><p>Because the central nervous system, heart, limbs, eyes, and ears are in their critical stages of development at this time, exposure to harmful substances called teratogens may lead to miscarriage or birth defects.</p><h4>About your pregnancy</h4><ul><li>
<a href="/Article?contentid=319&language=English">The First Month</a></li></ul> | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | https://assets.aboutkidshealth.ca/akhassets/Implantation_week4_MED_ILL_EN.jpg | The first trimester: Month one | | False | | | | | | | | |
