Fertilization, Implantation, and Embryonic Development

Fertilization is the fusion of a sperm and an egg to form a zygote, which divides into an embryo, implants in the wall of the uterus and develops into a fetus over about 38 weeks of pregnancy. In brief: fertilization usually occurs in the oviduct, the embryo implants about 6 to 10 days later, the placenta provides oxygen and nutrients, and birth occurs at around 40 weeks of pregnancy counted from the last menstrual period. From the end of the eighth week of development, the embryo is called a fetus.

Human development before birth takes place in stages, and the placenta links mother and baby. This guide follows development from the zygote to birth, explains the structure and role of the placenta and umbilical cord, covers the stages of pregnancy, includes a worked example on dating a pregnancy and corrects common mistakes.

How does fertilization happen?

After sexual intercourse, millions of sperm enter the vagina, and only a few hundred reach the oviduct. When a sperm reaches the egg, the enzymes in its acrosome digest a path through the layers surrounding the egg. The membranes of the sperm and egg fuse, and the sperm nucleus enters the egg. Changes in the egg coat then block other sperm, which prevents the entry of more than one.

The two nuclei, each with 23 chromosomes, fuse to form a single nucleus with 46 chromosomes. The new cell is the zygote. The sex of the child is determined at this point: the egg always carries an X chromosome, and the sperm carries either an X (giving a girl, XX) or a Y (giving a boy, XY).

What happens to the zygote?

The zygote divides by mitosis about 24 to 30 hours after fertilization, and then divides repeatedly while it is moved along the oviduct. The divisions are called cleavage, and the cells get smaller, not larger.

• 2 to 4 days: a solid ball of 16 or so cells, called the morula.
• About 5 days: a hollow ball called the blastocyst, with an outer layer of cells that will form the placenta and an inner cell mass that will form the embryo. The inner cells are stem cells that can become any cell type.

Twins arise in two ways. Identical (monozygotic) twins form when one embryo splits into two, and they have the same DNA. Non-identical (dizygotic) twins form when two eggs are released and each is fertilized by a different sperm, so they are as similar as other siblings.

What is implantation?

The blastocyst reaches the uterus about 5 to 6 days after fertilization and attaches to the lining. Outer cells release enzymes that digest a small area, and the blastocyst sinks into the thick lining, a process completed by about day 10 to 12. Cells around the embryo begin to release hCG, which signals the ovary to keep producing progesterone, as described in the article on the menstrual cycle.

Not all fertilized eggs implant. A large proportion of early embryos are lost, often because of chromosomal abnormalities, and this frequently happens before a woman is aware of the pregnancy.

What do the placenta and umbilical cord do?

The placenta develops from both the embryo and the uterine lining and is fully functional by about week 12. It is a disc-shaped organ, about 20 centimeters wide at the end of pregnancy. It contains many finger-like villi from the embryo, which lie in pools of the mother’s blood. The mother’s blood and the fetus’s blood come very close, but do not mix, and are separated by a thin barrier.

• Exchange: oxygen, glucose, amino acids, vitamins and minerals diffuse or are transported from the mother to the fetus, and carbon dioxide and urea pass from the fetus to the mother.
• Antibodies: some of the mother’s antibodies cross the placenta and give the baby temporary immunity.
• Hormones: the placenta makes progesterone and estrogen that maintain the pregnancy.
• Barrier: it blocks many bacteria, but some viruses and substances such as alcohol, nicotine and certain drugs can cross it and harm the fetus.

The umbilical cord joins the fetus to the placenta. It contains two arteries that carry oxygen-poor blood and waste from the fetus to the placenta, and one vein that carries oxygen-rich blood and nutrients to the fetus. The embryo is also surrounded by the amniotic sac with amniotic fluid, which cushions it, keeps the temperature steady and allows movement.

What are the stages of pregnancy?

Pregnancy is divided into three trimesters of about 13 weeks each.

• First trimester: the main organs form between weeks 3 and 8. This is the time when the embryo is most sensitive to harmful substances. By week 12, the fetus has recognizable limbs, fingers and facial features, and is about 6 centimeters long.
• Second trimester: the fetus grows rapidly, and movements can be felt from about weeks 16 to 20. The fetus develops hair and a covering of fine fat-containing material.
• Third trimester: the lungs mature, the fetus gains weight rapidly and usually turns head down. A baby born at term typically weighs about 3 to 3.5 kilograms.

Key stages of human development

How does birth occur?

Birth begins with signals that are not fully understood, but involve hormones from the fetus and the mother. Contractions of the uterus start, and positive feedback with the hormone oxytocin makes them stronger. In the first stage, the cervix opens (dilates) to about 10 centimeters. In the second stage, the baby is pushed out through the vagina, and in the third stage the placenta (afterbirth) is expelled. The umbilical cord is clamped and cut. After birth, the newborn starts to breathe and the lungs inflate.

Worked example: calculating a due date

Doctors usually count pregnancy from the first day of the last menstrual period (LMP), not from fertilization, because the day of fertilization is usually not known. A common rule (Naegele’s rule) adds 280 days (40 weeks) to the LMP.

1. Suppose the last menstrual period began on 10 March.
2. Naegele’s rule: add one year, subtract three months and add seven days. 10 March plus one year is 10 March of the next year; minus three months is 10 December; plus seven days is 17 December.
3. Counting 280 days directly: days left in March after the 10th: 21. Running totals at the end of each month: April 51, May 82, June 112, July 143, August 174, September 204, October 235, November 265. The remaining 15 days (280 − 265) give 15 December.
4. Fertilization date: it occurs about 14 days after the start of the last period, around 24 March. From then to 15 December is 266 days, which is 38 weeks.

The two methods differ by two days (17 December and 15 December), because Naegele’s rule uses calendar months of unequal length. Only about 4 to 5 percent of babies are born on the due date, and a birth between 37 and 42 weeks is considered normal. Ultrasound measurements early in pregnancy are often used to correct the date.

How was development studied?

Aristotle observed the development of chick embryos. In 1827 Karl Ernst von Baer discovered the mammalian egg. The first successful birth after in vitro fertilization (IVF), Louise Brown, took place in England in 1978, following work by Robert Edwards and Patrick Steptoe, and Edwards received the Nobel Prize in Physiology or Medicine in 2010. Ultrasound imaging of the fetus has been routine since the 1970s and 1980s.

Which mistakes are common?

Mistake 1: Saying that mother’s and baby’s blood mix in the placenta

They are separated by a thin barrier, and substances are exchanged by diffusion and transport.

Mistake 2: Mixing up embryo and fetus

The embryo stage covers about weeks 2 to 8 of development, and after this the developing baby is called a fetus.

Mistake 3: Counting pregnancy from the date of fertilization

Doctors count from the last menstrual period, so pregnancy lasts about 40 weeks by that count, or 38 weeks from fertilization.

Mistake 4: Thinking that the placenta protects the fetus from everything

Alcohol, nicotine and some drugs and viruses pass through it.