Fertilization: your pregnancy week by week

Fertilization of the egg with sperm generally occurs during 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 article provides a summary of each stage of pregnancy, what to expect, and insights into how your baby is developing.

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 after sexual intercourse, sperm must be ejaculated into the vagina so that it can travel through the opening of the cervix and into the fallopian tubes.^2,3

Once in the fallopian tubes, the sperm will penetrate the egg and complete fertilization.² During the third week after the first day of your last period, your fertilized egg moves along the fallopian tube towards the 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 gender and physical characteristics, as well as 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 mother's egg is always the same and is known as the X chromosome, but the sex chromosome from the father's sperm may be an X or a Y chromosome.

If the egg is fertilized by a sperm containing an X chromosome, the baby will be a girl (XX). If the sperm contains a Y chromosome, the baby will be a boy (XY).

At 3 weeks pregnant, baby 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 is comprised of 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

References:

1.        Mayo Clinic, Fetal development: The 1st trimester, accessed 4 July 2015.

2.         NLM, Fetal development, accessed 4 July 2015.

3.         American Pregnancy Association, Pregnancy week 3, accessed 4 July 2015.

4.         Embryology, Morula, accessed 4 July 2015.