Week 2

A new life – a unique human individual – is formed at conception, also called fertilization, when a man’s sperm fuses with a woman’s egg inside the fallopian tube. The resulting single-celled human embryo is called a zygote.⁶ Both the sperm and the egg were alive, but each only contained 23 chromosomes, half the genetic information needed to create a new human being.

The zygote's DNA, packaged into chromosomes, is present on Day 1 and has every instruction the embryo needs to develop, grow and become an adult. The DNA sequence contains astounding genetic variation and serves as the foundation for a person’s biological individuality, including sex, eye color, height, and many other traits. Individuals with a pair of XX sex chromosomes are female and those with a pair of XY sex chromosomes are male.⁷

There is enough genetic variation that no two humans have been, nor ever will be, genetically identical.⁸ In fact, the probability that two non-twin siblings have the same combination of chromosomes is one in 7 trillion!⁹ Even identical twins, who originate from the same fertilized egg, develop small genetic differences that make each genetically unique.¹⁰

Out of the hundreds of millions of sperm released during ejaculation, only about 1% make it into the uterus. The journey from the cervix to the fallopian tube can take anywhere from 30 minutes to six days. In contrast, the egg moves through the fallopian tube with the help of gentle, wave-like motions created by tiny hair-like structures called cilia.¹¹

Between 12 and 30 hours after the egg and sperm are joined, the embryo’s DNA replicates, forming an exact copy. Then the two identical copies of chromosomes move to opposite sides of the cell, and the cell divides, forming two cells. Within 40 hours of fertilization, the two new cells divide again creating a four-celled human being.¹⁵ The cells of the early embryo divide rapidly, each time doubling the total number of cells. The embryo consists of stem cells capable of becoming any of the tissues and organs in the human body. Interestingly, with each division, the cells gets smaller and smaller allowing each new set to fit within the original zona pellucida as shown in the picture below. This early embryo, now called a morula, resembles a tiny raspberry.¹⁶

Even as the embryo’s cells keep dividing, tiny hair-like structures in the fallopian tube gently sweep the developing embryo toward the uterus. By about five days after conception, the growing ball of cells has hollowed out and become a blastocyst. To implant in the uterus, the blastocyst must first shed the protective outer layer called the zona pellucida by releasing specialized enzymes.¹⁷ Only then can the young embryo begin to embed within the uterine lining. Remarkably, some research suggests that the embryo may begin communicating biochemically with the mother’s body even before implantation, signaling the unborn baby’s presence and possibly preparing the uterus for arrival.^{18 19 20}

About six days after fertilization, the early embryo begins to implant in the uterus. The outer cells in the blastocyst have special sticky molecules that help the blastocyst bind to the wall of the uterus.²¹ The wall of the uterus will start providing nutrients to the developing embryo as the embryo embeds.²² The unborn child is now in his permanent home, safe and secure in his mother’s womb, and will stay there until ready for birth.