Following fertilization of the ovum, a zygote is formed, and in this first week there are a number of important cellular activities that begin in the uterine tube. At the end of this first week, the zygote enters the uterus and implantation occurs.
The second through the eighth week are the embryonic stage. It is the stage in prenatal development between the zygote and the fetus. During this time, the embryo increases in length from about 1.5 mm to 23 mm. Certain layers of the embryonic disk give rise to the principal organ systems, and the embryo begins to show the human form.
It is important to recognize that at this stage, while the baby’s organs are being formed, the embryo is particularly sensitive to the effect of toxic drugs and chemicals. Scientific studies have confirmed that Zofran (ondansetron) does cross the placental barrier from the mother to the embryo during this period. Furthermore, and because Zofran is a lipid-based drug, it crosses the placental barrier in relatively high concentrations and was found in all tissues sampled. In fact, the average fetal/maternal ratio was 41% (i.e., the level of Zofran in the forming baby was nearly half of the level of Zofran in the mother).
The third stage is the fetal period, and this stage extends from the beginning of the ninth week until birth. The main features of this period are the growth and differentiation of those tissues and organs that began their development in the embryonic period. During this stage the fetus begins to move and the life-sustaining reflexes such as sucking and swallowing are established.