How we are born

Abstract

Human reproduction is relatively inefficient. Nearly 30% of pregnancies result in spontaneous losses, which are both a clinical problem and a psychological stress to the families involved. Furthermore, although the human population is growing rapidly and is predicted to reach 9 billion by 2050, 15% of couples worldwide are childless because of infertility. Many underlying causes of infertility have been overcome by assisted reproductive technologies such as in vitro fertilization, yet pregnancy success rates using such approaches remain disappointingly low. Since mechanistic approaches to study human reproductive processes are ethically restricted, future advances in fertility treatment and the development of new contraceptives rely predominantly on the study of the factors influencing reproduction in model systems. The articles in this Reproductive Biology Review series present updates on the current understanding of various reproductive processes in model systems and raise questions that need to be addressed if we are to improve human reproductive health.

Figure 1. Timeline of pregnancy events in humans.

This timeline is an approximation based on a 28- to 30-day menstrual cycle, the average length of a woman’s menstrual cycle. A surge in luteinizing hormone (LH) levels occurs at approximately day 13 of the menstrual cycle, and this stimulates ovulation. The ruptured follicle forms a corpus luteum, which produces large amounts of progesterone that modify the uterus lining so that it is receptive to implantation of an embryo if fertilization has occurred. Fertilization takes place in the fallopian tube. As the fertilized egg travels down toward the uterus, it undergoes a series of early cleavage divisions, producing increasing numbers of progressively smaller cells, known as blastomeres, without changing the overall size of the embryo. The 16-cell–stage embryo is known as a morula, and its cells are indistinguishable. As further cleavage steps occur and the blastocoel develops, the morula becomes a blastocyst. The blastocyst implants into the uterine lining approximately 6–8 days after ovulation. A fully functional placenta does not form until approximately 10 weeks after ovulation. Parturition at full term occurs 40 weeks after ovulation.