Sperm, Eggs, Fertilisation and Implantation

Fertilisation and implantation are separate events. Sperm must reach an oocyte, one sperm must fuse with it, the resulting cells must divide, and the developing blastocyst must reach and attach to the uterine lining.

Reproductive cells contain half the usual chromosome number Most human body cells contain 46 chromosomes arranged in 23 pairs. Sperm and oocytes are gametes. Each normally contains 23 individual chromosomes. When a sperm and oocyte combine, their chromosome sets join. The first new cell usually contains 46 chromosomes. Gametes are produced through a specialised form of cell division called meiosis. Meiosis reduces the chromosome number and creates genetic variation. Each sperm and oocyte is therefore genetically different from most others. Chromosome number can sometimes differ from the usual pattern. These differences may prevent development, cause pregnancy loss or result in a chromosomal condition. Oocyte development begins before birth The cells that can later become mature oocytes begin developing before birth. At birth, the ovaries already contain immature oocytes within follicles. The clinically relevant ovarian reserve is established before birth and declines over time. Menstrual cycles recruit oocytes from this existing reserve rather than creating a new monthly supply. After puberty, hormone signals allow groups of follicles to begin developing during menstrual cycles. Usually one follicle becomes dominant. During ovulation, it releases a secondary oocyte. The word egg is commonly used for this cell. Strictly, it completes its final meiotic division only if a sperm enters it. The number of available oocytes declines with age. The proportion with chromosome abnormalities also rises, which contributes to reduced fertility and increased miscarriage risk, although age does not predict one individual's fertility with certainty. Sperm production begins at puberty Sperm production begins at puberty within the seminiferous tubules of the testes. The process produces large numbers of small cells with heads, middle sections and tails. The head contains the chromosomes and specialised structures needed to interact with an oocyte. The middle section supplies energy, while the tail supports movement. New sperm continue to mature in the epididymis. Sperm quality can be influenced by illness, medicines, hormones, genetic factors and environmental exposures. One semen sample does not represent every future sample or every part of fertility. Sperm can be present before ovulation After ejaculation into the vagina, sperm can travel through the cervix and uterus towards the uterine tubes. Many sperm do not survive this journey. Only a small proportion reach the region where an oocyte may be present. Supportive cervical mucus near ovulation helps sperm movement. Sperm can remain capable of fertilisation for several days within suitable reproductive tract conditions. The released oocyte remains fertilisable for a much shorter period, usually around one day. Pregnancy can therefore result from sex occurring before ovulation rather than only on the day an oocyte is released. Calendar calculations cannot identify this fertile period with certainty. Fertilisation usually occurs within a uterine tube Fertilisation most often occurs in the wider part of a uterine tube. Before a sperm can fuse with an oocyte, it undergoes functional changes within the reproductive tract. This process is called capacitation. The sperm must pass through layers surrounding the oocyte. Usually one sperm fuses with the oocyte cell membrane. The oocyte then changes rapidly to reduce the chance of additional sperm entering. The sperm and oocyte chromosome sets come together. The resulting first cell is called a zygote. Fertilisation does not occur inside the ovary. It also does not usually occur after the developing cells have reached the uterus. Early cells divide while travelling The zygote begins dividing without immediately growing larger overall. It divides into two cells, then four and then more cells. This early division is called cleavage. The developing group of cells travels through the uterine tube towards the uterus. It forms a compact cluster called a morula and later a hollow structure called a blastocyst. The blastocyst contains an inner cell mass, which gives rise to the embryo proper, and an outer trophoblast layer. The trophoblast contributes to the fetal part of the placenta, while maternal uterine tissue also contributes to placental development. This journey takes several days. Normal movement through the uterine tube is important. Implantation is a separate step Implantation begins when a blastocyst attaches to a receptive endometrium. The outer trophoblast cells interact with and invade the uterine lining, beginning to establish the structures that later support exchange through the placenta. Implantation usually begins several days after fertilisation. Fertilisation therefore does not mean implantation has already happened. Not every fertilised oocyte develops into a blastocyst. Not every blastocyst implants, and not every implanted pregnancy continues. Some fertilised oocytes do not develop to the blastocyst stage, some blastocysts do not implant, and some pregnancies end before a person knows they are pregnant. When an early pregnancy loss occurs, it is usually not caused by something the person did or did not do. Many early miscarriages result from chance chromosome abnormalities. Early pregnancy hormones begin after implantation After implantation begins, developing placental tissue produces human chorionic gonadotrophin, usually shortened to hCG. hCG maintains the corpus luteum so that progesterone production continues during early pregnancy. Pregnancy tests detect hCG in urine or blood. They do not directly detect sperm, fertilisation or implantation. A test taken immediately after sex cannot show whether pregnancy will occur. Testing too early can produce a negative result because hCG has not risen enough to be detected. Most home pregnancy tests are most reliable from the first day of a missed period. If you do not know when the next period is due, NHS guidance advises testing at least 21 days after the last unprotected sex. A negative result taken earlier cannot reliably exclude pregnancy. If the result is negative but pregnancy is still possible, repeat the test after a few days and follow the test instructions. Implantation normally occurs inside the uterus A pregnancy developing outside the uterine cavity is called an ectopic pregnancy. Most ectopic pregnancies develop within a uterine tube. An ectopic pregnancy cannot develop normally and can cause serious internal bleeding. A person may have an ectopic pregnancy before receiving a positive home pregnancy test. Contact a GP or NHS 111 promptly if you might be pregnant and have one sided lower abdominal pain, vaginal bleeding or brown watery discharge, shoulder tip pain, or pain when passing urine or stool. Seek advice even if you have not had a positive pregnancy test. Call 999 or go to A&E immediately for sharp, sudden and intense abdominal pain with severe dizziness, fainting, sickness or marked pallor. Heavy bleeding accompanied by severe pain, faintness, collapse or feeling very unwell also requires emergency help. Assisted reproduction changes parts of the route During in vitro fertilisation, oocytes are collected and mixed with sperm in a laboratory. If fertilisation occurs, embryos are observed for several days. One or more embryos may then be transferred into the uterus. Fertilisation therefore occurs outside the body during IVF, but implantation must still occur within the uterine lining for pregnancy to become established. Embryo transfer does not guarantee implantation or live birth.

Fertilisation usually occurs in a uterine tube, while implantation occurs later in the uterine lining. Pregnancy tests detect hCG after implantation rather than detecting fertilisation directly.