When semen is deposited in the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes. Fertilization of the ovum (egg cell) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its genetic material (DNA). The egg then polarizes, repelling any additional sperm. The resulting combination is called a zygote. The term "conception" refers variably to either fertilization or to formation of the conceptus after uterine implantation, and this terminology is controversial.
Prior to fertilization, each ovum contains a complete human genome, including a single X but no Y chromosome. Likewise, each spermatozoon contains a complete set of autosomes and a single sex chromosome, either X or Y. The resulting zygote is similar to the majority of somatic cells because it contains two copies of the genome in a diploid set of chromosomes. One set of chromosomes came from the nucleus of the ovum and the second set from the nucleus of the sperm. If the spermatozoon contributes a Y chromosome then the zygote will develop as a male. Unlike the X chromosome, the Y chromosome contains very little genetic information. However it does contain a gene, SRY, which will switch on androgen production at a later stage, leading to the development of a male body type. In contrast, the mitochondrial genetic information of the zygote comes entirely from the mother via the ovum.
The embryonic period in humans begins at fertilization (2nd week of gestation) and continues until the end of the 10th week of gestation (8th week of development).
The zygote spends the next few days traveling down the Fallopian tube. Meanwhile it divides several times to form a ball of cells called a morula. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst. Up to this point there is no growth in the overall size of the embryo, so each division produces successively smaller cells.
The blastocyst reaches the uterus at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida, a glycoprotein shell, occurs. This is required so that the trophectoderm cells, which give rise to extra-embryonic structures such as the placenta, of the blastocyst can come into contact with the luminal epithelial cells of the endometrium. It then adheres to the uterine lining and becomes embedded in the endometrial cell layer. This process is also called implantation. In most successful pregnancies, the conceptus implants 8 to 10 days after ovulation . The inner cell mass forms the embryo, while the outer cell layers form the membranes and placenta. Together, the embryo and its membranes are referred to as a conceptus, or the "products of conception".
Rapid growth occurs and the embryo's main external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or miscarriage, in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing embryo is also susceptible to toxic exposures, such as:
- Alcohol, certain drugs, and other toxins that cause birth defects, such as Fetal alcohol syndrome
- Infection (such as rubella or cytomegalovirus)
- Radiation from x-rays or radiation therapy
- Nutritional deficiencies such as lack of folate which contributes to spina bifida
Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny." For example, the backbone is a common structure among all vertebrates such as fish, reptiles and mammals, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.
The fetal period begins at the end of the 10th week of gestation (8th week of development). Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and by a list of changes by weeks of gestational age.
Because the precursors of the organs are formed, fetus also is not as sensitive to damage from environmental exposures as the embryo. Instead, toxic exposures often cause physiological abnormalities or minor congenital malformation.
From the 8th week until birth (around 38 weeks), the developing organism is called a fetus. The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation. All major structures are already formed in the fetus, but they continue to grow and develop.