Pregnancy Week 3 and 4

The third week of pregnancy is when the development of the zygote starts after conception. Most women will not know if they are pregnant until the end of week 4 or during week 5 when they miss their next period. During weeks 3 and 4, the fertilized egg travels down to the uterus as a dividing mass of cells, which eventually implants into the endometrium and starts pregnancy.

What happens during Week 3 of pregnancy?

If the ovulation and fertilization occurred on week 3, then it can be said that conception has occurred. About 30 hours after the egg is fertilized, it divides into two cells, at which point it is called a zygote. The cells of the zygote continue to divide as it moves down the fallopian tube towards the uterus. It takes about 3-4 days for the zygote to reach the uterus after fertilization. It continues to stay as a dividing mass of cells that floats around the uterus for 2-3 days.

Soon after conception takes place, the hormones in the woman’s body change giving rise to some of the mild pregnancy third-week symptoms some women experience. The level of progesterone increases, and there is also an increase in the levels of human chorionic gonadotropin (hCG). The hCG is also the hormone the pregnancy tests look for to determine a positive pregnancy. However, it is too early to tell by home pregnancy tests since the hCG levels are too low to detect.

What happens during Week 4 of pregnancy? 

By the end of the 4^th week, the floating mass of dividing cells attaches itself to the uterine wall in a process termed implantation. At this stage, it is called a blastocyst. It is a lump of about 200 cells that is 0.2mm wide and made up of multiple layers of cells. The outermost, later called ectoderm, would eventually develop into the brain and nervous system. The middle layer, which is called the mesoderm, will develop into the heart, blood vessels, bones and muscles. The innermost layer called the endoderm will develop into the digestive and respiratory systems. The blastocyst also has an outer layer called chorionic villi, which are tentacles like appendages that will develop into the placenta.

The fourth week of pregnancy is usually when the implantation occurs. The process starts about 6 days after the egg is fertilized and takes 3-4 more days to reach completion. As the blastocyst burrows itself into the endometrium, some women experience light cramping and the 4-week implantation bleeding also known as spotting. Spotting is sometimes mistaken for a period initially as it occurs at about the same time the period was supposed to arrive. Women don’t usually notice any signs of pregnancy until the third week. It is only when the period on the fourth week is missed that they get their first sign of pregnancy.

Wrapping Up

Weeks 3 and 4 is when pregnancy moves from conception to implantation. Women are often unaware of the processes and only realize they are pregnant when they miss their next period. Some of the symptoms that can occur late into the 4^th week include feeling bloated, tired, nausea and mood swings.