In vitro fertilisation, commonly known as IVF, is a medical procedure where an egg is fertilised by sperm outside the human body, typically in a laboratory dish. The term "in vitro" comes from Latin, meaning "in glass," referring to the …
In a natural menstrual cycle, a woman's body typically matures and releases one egg per month. For IVF, doctors administer synthetic hormones—primarily follicle-stimulating hormone (FSH) and luteinizing hormone (LH)—to deliberately override this single-egg limit. These injections, given daily over 8-14 days, stimulate multiple follicles in the ovaries to develop simultaneously, each containing an egg.
The hormone regimen follows a carefully timed protocol. Patients first receive medications to suppress their natural cycle, preventing premature ovulation. Then the stimulation hormones prompt 10-20 follicles to mature at once, dramatically increasing the number of eggs available for retrieval. Doctors monitor this process through blood tests measuring estrogen levels and ultrasound scans showing follicle growth.
When follicles reach optimal size—typically 18-20 millimeters in diameter—a final "trigger shot" of human chorionic gonadotropin (hCG) is administered. This hormone mimics the natural ovulation signal and causes the eggs inside follicles to complete their final maturation step over the next 36 hours, preparing them for retrieval.
Egg retrieval occurs exactly 34-36 hours after the trigger shot, timed to catch eggs at peak maturity before natural ovulation would release them. The patient receives sedation or light anesthesia for this outpatient procedure. Using transvaginal ultrasound for guidance, the physician inserts a thin, hollow needle through the vaginal wall and into each visible ovarian follicle.
The needle punctures each follicle and applies gentle suction to draw out the follicular fluid, which contains the egg. This fluid is immediately passed to an embryologist who examines it under a microscope to locate and isolate each egg. The entire process typically takes 20-30 minutes and retrieves anywhere from 8-15 eggs on average, though numbers vary considerably based on individual response to stimulation.
Not all retrieved eggs will be usable—some may be immature or abnormally developed. The embryologist assesses each egg's maturity by examining the surrounding cells and the egg's appearance. Only mature eggs at the right developmental stage, called metaphase II oocytes, are suitable for fertilization.
On the same day as egg retrieval, a sperm sample is prepared in the laboratory. Technicians wash and concentrate the sample, separating healthy, motile sperm from seminal fluid and debris. For standard IVF, approximately 50,000-100,000 prepared sperm are placed in a petri dish with each mature egg, allowing fertilization to occur naturally as sperm compete to penetrate the egg's protective outer layer.
When sperm quality or quantity is poor, embryologists use intracytoplasmic sperm injection (ICSI). This technique involves selecting a single healthy sperm and injecting it directly through the egg's outer membrane and into the cytoplasm using a microscopic glass needle. ICSI bypasses the sperm's need to penetrate on its own, making fertilization possible even with severely compromised sperm.
Within hours after sperm and egg unite, the fertilization process begins. The egg completes its final cell division and the genetic material from sperm and egg combine to form a single-celled embryo with a complete set of chromosomes. By the next morning—about 16-18 hours later—embryologists check for signs of successful fertilization, looking for two distinct structures called pronuclei that contain the genetic contributions from each parent.
Fertilized eggs are placed in specialized incubators that precisely control temperature (37°C), humidity, and gas composition to replicate the environment of the fallopian tube and uterus. The culture medium—a carefully formulated liquid solution—provides nutrients, amino acids, and proteins that embryos need for energy and growth. Embryologists monitor development daily, observing cell division without disturbing the controlled environment.
During the first three days, the embryo divides from one cell to two, then four, then eight cells, with each cell called a blastomere. Around day four, the embryo becomes a compact ball of cells called a morula. By day five or six, it develops into a blastocyst—a more complex structure with about 100-200 cells organized into an inner cell mass (which will become the fetus) and an outer layer called the trophectoderm (which will form the placenta).
Extended culture to the blastocyst stage allows embryologists to identify which embryos have the greatest developmental potential, as many embryos that look healthy at day three fail to progress further. Some clinics also perform preimplantation genetic testing during this stage, removing a few cells for chromosomal analysis. This testing can identify embryos with the correct number of chromosomes, improving the chances of successful pregnancy.
Embryo transfer typically occurs 3-5 days after fertilization, with many clinics preferring day-five blastocysts when possible. The patient arrives with a comfortably full bladder, which helps position the uterus optimally and provides better ultrasound visualization. Unlike egg retrieval, this procedure requires no anesthesia—most women describe it as similar to a pap smear.
The embryologist loads one or occasionally two selected embryos into a soft, flexible catheter—a thin tube barely wider than a strand of spaghetti. Using a speculum to visualize the cervix and abdominal ultrasound for guidance, the physician gently threads this catheter through the cervical opening and into the uterine cavity. When the catheter tip reaches the ideal position—typically 1-2 centimeters from the top of the uterus—the embryologist depresses the syringe plunger to release the embryo in a tiny drop of culture medium.
After withdrawing the catheter, the embryologist immediately examines it under a microscope to confirm the embryo was successfully deposited and didn't stick to the catheter walls. The patient rests briefly, then can resume normal activities. The embryo floats freely in the uterine cavity for 1-2 days before hatching from its protective shell and beginning to implant into the endometrial lining, where pregnancy either establishes or fails based on embryo quality and uterine receptivity.