Concepts: Fertilisation, pregnancy, parturition and lactation

4.1 From insemination to fertilisation

During copulation, semen is released into the vagina, which is insemination. The motile sperms swim through the cervix, pass through the uterus and reach the ampulla of the fallopian tube. The ovum that the ovary released is also swept along to the same swelling. Fertilisation can happen only if the ovum and the sperms arrive there at the same time, which is why not every copulation leads to pregnancy.

Fertilisation is the joining of one sperm and one ovum. When a sperm contacts the zona pellucida, it sets off changes in the egg membrane that stop any further sperm from entering. That block guarantees that only one sperm fertilises an egg. The enzymes released from the acrosome digest a path through the zona pellucida and the plasma membrane, and the sperm enters the cytoplasm.

Entry of the sperm restarts the second meiotic division of the secondary oocyte. This division is unequal too, yielding a haploid ovum, or ootid, plus a second polar body. The haploid nucleus of the sperm now fuses with the haploid nucleus of the ovum, and the result is a diploid zygote with 46 chromosomes.

4.2 The sex of the baby is fixed at fertilisation

The chromosome pattern is XX in the human female and XY in the male. Every ovum therefore carries an X chromosome, while a sperm may carry either X or Y. Half of the sperms carry X and half carry Y. If an X-bearing sperm fertilises the ovum the zygote is XX and becomes a girl, and if a Y-bearing sperm fertilises it the zygote is XY and becomes a boy. The sex of the child is therefore decided by the father's sperm and not by the mother, and the decision is already made at fertilisation.

4.3 Cleavage and implantation

The zygote begins mitotic division, called cleavage, as it travels through the isthmus of the oviduct towards the uterus. The divisions produce 2, then 4, then 8, then 16 cells, and these cells are the blastomeres. An embryo with 8 to 16 blastomeres is a morula. As the morula travels deeper into the uterus it divides further and turns into a blastocyst.

The blastocyst has two cell populations. The outer layer is the trophoblast, and inside it, attached to the trophoblast, is the inner cell mass. The trophoblast adheres to the endometrium while the inner cell mass gives rise to the embryo. Once attached, uterine cells multiply quickly and grow across the blastocyst, so the blastocyst becomes buried in the endometrium. That embedding is implantation, and it leads to pregnancy.

4.4 The placenta

After implantation, finger-like projections called chorionic villi appear on the trophoblast. Uterine tissue and maternal blood surround them, and the chorionic villi and the uterine tissue interlock to form the placenta. The placenta is the structural and functional link between the embryo, or foetus, and the mother.

It carries out two kinds of work. It exchanges materials: oxygen and nutrients pass to the embryo, and carbon dioxide and excretory waste pass back to the mother. The umbilical cord connects the placenta to the embryo and carries these substances. The placenta also works as a gland, releasing progestogens, oestrogens, hCG (human chorionic gonadotropin) and hPL (human placental lactogen). In the later part of pregnancy the ovary secretes a hormone called relaxin. hCG, hPL and relaxin appear in a woman only during pregnancy. The maternal blood also carries several times the usual levels of oestrogens, progestogens, cortisol, prolactin and thyroxine, because these hormones support fetal growth, change the mother's metabolism and maintain the pregnancy.

4.5 The embryo month by month

Immediately after implantation, the inner cell mass separates into an outer ectoderm and an inner endoderm, and a mesoderm soon appears between them. Every tissue and organ of the adult traces back to these three layers. The inner cell mass also contains stem cells, cells that can still become any tissue.

Human pregnancy lasts about nine months. At one month the heart is already formed, and a stethoscope can pick up its beating as the earliest sign of the growing foetus. Two months on, limbs and digits have appeared. At the close of 12 weeks, which is the end of the first trimester, the major organ systems exist and the limbs and external genitalia are well formed. The foetus first moves and grows hair on its head during the fifth month. Near 24 weeks, the close of the second trimester, fine hair covers the body, the eyelids have opened apart and eyelashes are present. By the end of nine months the foetus is fully developed and ready for delivery.

4.6 Parturition

The average duration of human pregnancy, about nine months, is the gestation period. Childbirth is called parturition, and strong uterine contractions at the end of pregnancy bring it on.

The fully developed foetus and the placenta send the first signal, producing gentle contractions known as the foetal ejection reflex. This reflex makes the maternal pituitary release oxytocin. Oxytocin acts on the uterine muscle and produces stronger contractions, and stronger contractions release more oxytocin. That loop of contraction and hormone release keeps increasing the strength of the contractions until the baby is pushed out through the birth canal. The placenta is expelled soon after the baby.

4.7 Lactation

The mammary glands of the mother differentiate during pregnancy and start producing milk towards the end of it. The process is lactation, and it lets the mother feed the newborn. The milk produced in the first few days is colostrum. It carries several antibodies, and these are essential for building the newborn's resistance to infection. Doctors recommend breastfeeding during the early months of infant growth, and that advice rests on the antibody content of the first milk.