Fertilization of the egg with sperm generally occurs during the two weeks following the first day of your last menstrual period.1
The week of pregnancy that you are entering is dated from the first day of your last period. This means that in the first two weeks or so, you are not actually pregnant - your body will be preparing for ovulation as normal.
This MNT Knowledge Center feature is part of a series of articles on pregnancy. It provides a summary of each stage of pregnancy, what to expect, and insights into how your baby is developing. Take a look at the other articles in the series:
You will also see introductions at the end of some sections to any recent developments that have been covered by MNT's news stories. Also look out for links to information about related conditions.
Week 3: fertilization
You will ovulate (release an egg) around two weeks after the first day of your period (depending on the length of your menstrual cycle).
The fertilized egg (zygote) divides repeatedly as it moves down the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells. Then it becomes a hollow ball of cells called a blastocyst.
In order for fertilization to occur, sperm must be ejaculated into the vagina through sexual intercourse or otherwise be inserted through the opening of the cervix to travel into the fallopian tubes.2,3
Once in the fallopian tubes, the sperm will penetrate and fertilize the egg.2 During the third week after the first day of your last period, your fertilized egg moves along the fallopian tube towards your womb.
Once united, the egg and sperm form a zygote, which contains 46 chromosomes - 23 from the female and 23 from the male, which will ultimately determine the genetic make-up of your child.1-3
These chromosomes will determine the sex and physical characteristics of the fetus, and influence personality and intelligence.1,3
Chromosomes are tiny threadlike structures that each carry around 2,000 genes. Genes determine a baby's inherited characteristics, such as hair and eye color, blood group, height and build.
A fertilized egg contains one sex chromosome from its mother and one from its father. The sex chromosome from the egg is always the same and is known as the X chromosome, but the sex chromosome from the sperm may be an X or a Y chromosome.
If the egg is fertilized by a sperm containing an X chromosome, the fetus will be female (XX). If the sperm contains a Y chromosome, the fetus will be male (XY).
At 3 weeks pregnant, the fetus is the size of a pin head
The zygote's journey has only just begun, It will spend several days making its way down the fallopian tube, at which time it will develop into a morula, a ball of 12 to 15 cells and then into a blastocyst.1,2,4
The blastocyst, which at this point is rapidly multiplying, is a grouping of cells that contains an inner collection of cells. This blastocyst will ultimately develop into the embryo and an outer shell whose purpose is to provide protection and nourishment to the growing embryo.1,2
At this point, your future baby is still a cluster of cells measuring approximately .0019 inches, which is approximately the size of a pin head.3
During this time, it is important to speak with your health care provider regarding your current or desired exercise routine and your nutritional status. Increasing your consumption of folic acid and other vitamins may be recommended.3
Substances that can harm your growing baby and should be avoided include alcohol, illegal drugs, certain medications and foods. Caffeine and smoking should be discussed with your health care provider.
At this stage of pregnancy, there are very few expected symptoms however some women may have mild cramping and an increase in vaginal discharge during the ovulatory stage.3
Recent developments on fertilization from MNT news
In a new study published in the journal Biomicrofluidics, researchers from Taiwan reveal the creation of a new technique they say could eventually lead to more effective, cheaper in vitro fertilization for couples struggling to conceive.
Recently, UK members of parliament voted in support of legalizing mitochondrial donation - a form of IVF that could prevent severe genetic diseases being passed from mother to offspring. If the House of Lords votes in favor of the technique next month, the UK will be the first country to allow the creation of embryos from the DNA of three people. While supported by many, others say mitochondrial donation is a step too far for genetic engineering. We take a look at both sides of the argument and investigate the US standpoint on the procedure.
Genetic scientists have used a new technique that allows the whole genome of IVF embryos to be scanned via the cells of 10 biopsies. The researchers say the testing is the first to be able to detect all the new genetic mutations that happen uniquely in an individual, as opposed to only those that have been handed down from parents.