5 Stages of IVF: What Happens at Each Step

In vitro fertilization moves through five distinct stages: ovarian stimulation, egg retrieval, fertilization and embryo culture, embryo transfer, and luteal-phase support while you wait for implantation. Each stage involves different hormones, procedures, and decisions, and the choices made at one step ripple forward into the next. Understanding what actually happens in each phase, rather than just knowing the names, helps you ask better questions and feel less blindsided by the process.

Before Anything Starts

Before the five stages begin, your clinic runs baseline tests to figure out how your body is likely to respond to stimulation drugs. Two of the most important measurements are your anti-Müllerian hormone (AMH) level and your antral follicle count (AFC), which is the number of small, resting follicles visible on a transvaginal ultrasound. Both can be measured on any day of your cycle and together give the clinic a rough picture of your ovarian reserve, meaning how many eggs your ovaries can realistically produce in a stimulated cycle.¹ These numbers drive the dosing strategy for the next stage and help set realistic expectations for how many eggs might be retrieved.

Stage 1: Ovarian Stimulation

In a natural menstrual cycle your body typically matures and releases one egg. The goal of ovarian stimulation is to push multiple follicles to maturity at once so the clinic has more eggs to work with. You inject follicle-stimulating hormone (FSH) daily for roughly 8 to 14 days. A lower dose of FSH tends to produce fewer mature follicles, while a higher dose recruits more, but the upper limit is set by however many antral follicles were already sitting in your ovaries at the start.² More is not always better: pushing beyond what your ovaries can safely produce increases complications without adding mature eggs.

Alongside FSH, you also take a second drug to prevent your body from ovulating on its own before the eggs are collected. Two main protocols exist for this. A GnRH agonist “long protocol” starts the suppression drug before stimulation begins, while a GnRH antagonist protocol adds the suppression drug partway through stimulation. The antagonist approach is shorter and uses less medication overall, though historically the long agonist protocol has been considered better for follicle development and pregnancy rates in many patients.³ That picture is shifting, however. A retrospective study comparing both protocols in the same patients found that implantation and clinical pregnancy rates were higher with the antagonist protocol, and suggested it may be particularly helpful for patients who have already failed previous cycles.⁴ The antagonist protocol also carries a lower risk of ovarian hyperstimulation syndrome, which we will get to later.⁵

Throughout stimulation, you visit the clinic every couple of days for blood draws and ultrasounds. The clinic watches follicle sizes and hormone levels to decide when to trigger final maturation. The optimal follicle size at triggering is roughly 18 to 22 millimeters, but estrogen concentration also factors in. Triggering too early means immature eggs; triggering too late risks a premature LH surge that could cause you to ovulate and lose the eggs entirely.⁶ Once the trigger shot is given, egg retrieval is scheduled about 34 to 36 hours later.

Stage 2: Egg Retrieval

Egg retrieval is a short surgical procedure done under sedation. An ultrasound probe is placed in the vagina, and a thin needle is guided through the vaginal wall into each ovary. The needle connects to gentle suction, and follicles above about 12 millimeters are drained one by one, with the fluid sent immediately to the embryology lab to identify the eggs.⁷ The whole thing usually takes 15 to 30 minutes, and most people go home within a couple of hours.

Not every egg collected is usable. Embryologists assess each egg’s maturity under the microscope. Mature eggs, called metaphase II (M2) oocytes, are ready for fertilization right away. About 4% of eggs are at an earlier stage (metaphase I) and may mature in the lab over several hours before they can be used.⁸ Some eggs are too immature to use at all. Age plays a significant role here: in patients under 40, mature M2 oocytes produce good-quality embryos about 64% of the time, but that figure drops to around 27% in patients over 46.⁹ This steep decline with age is one of the main reasons clinics encourage earlier treatment when possible.

Stage 3: Fertilization and Embryo Culture

Once the mature eggs are identified, fertilization happens in one of two ways. In conventional insemination (CI), sperm are placed in a dish with the egg and allowed to fertilize it on their own. In intracytoplasmic sperm injection (ICSI), a single sperm is injected directly into the egg. ICSI was developed for male-factor infertility, where sperm count, motility, or shape makes natural fertilization unlikely. It has since become widely used even in cases with no male factor, but the evidence suggests that is not always helpful. When there is no male-factor issue, ICSI does not improve fertilization rates or embryo quality compared to conventional insemination and may even produce a slightly lower proportion of chromosomally normal embryos.¹⁰ When there is a male factor, ICSI fertilization rates come in around 74%, compared to roughly 77% for conventional insemination in non-male-factor cases.¹¹

After fertilization, embryos are cultured in the lab for three to six days. Early-stage embryos (day 3) are a clump of about six to eight cells. If a clinic has enough good-quality embryos at that point, they may extend culture to day 5 or 6, when the embryo has developed into a blastocyst, a structure of roughly 100 or more cells with distinct inner and outer layers.¹² Culturing to blastocyst stage helps the lab identify which embryos have the strongest developmental potential, since not all day-3 embryos survive the next two days. The trade-off is that patients who start with very few embryos risk having none survive to day 5.

Genetic Testing of Embryos

If you opt for preimplantation genetic testing for aneuploidy (PGT-A), the lab takes a small biopsy of the blastocyst’s outer cell layer (the trophectoderm) before freezing. This involves removing roughly 5 to 7 cells and analyzing their chromosomes.¹³ A systematic review and meta-analysis found that trophectoderm biopsy is the most accurate method currently available for checking an embryo’s genetic status, but it still isn’t perfect, with a sensitivity around 84% and a specificity around 79%.¹⁴ This means a small percentage of embryos will be labeled abnormal when they are actually fine, or labeled normal when they carry a problem. PGT-A is most commonly recommended for patients over 35 or those with recurrent miscarriage or failed transfers, and it adds cost and a freeze cycle to the timeline.

Time-Lapse Monitoring and AI

Traditionally, embryologists check on developing embryos by briefly removing them from the incubator and placing them under a microscope. Time-lapse monitoring systems replace these interruptions with a camera inside the incubator that continuously photographs the embryos, keeping culture conditions stable while capturing a detailed record of how each embryo divides and grows.¹⁵ This wealth of image data has opened the door to artificial intelligence tools that score embryos automatically. Across studies published between 2020 and 2025, AI-based assessments of embryos consistently outperformed traditional grading by embryologists, with reported prediction accuracies for pregnancy or live birth in the range of 70% to 86%.¹⁶ These tools are still being refined and validated, but they represent a meaningful shift away from the subjective “eyeball” scoring that has been the standard for decades.

Stage 4: Embryo Transfer

Embryo transfer is the simplest procedure of the five stages in terms of what you physically experience. You lie on an exam table with a moderately full bladder (which helps the ultrasound image). The doctor inserts a thin, soft catheter through your cervix and deposits the embryo, suspended in a tiny drop of culture medium, into the middle of your uterine cavity.¹⁷ There is no anesthesia involved. The process takes only a few minutes, and the catheter is so fine that many people describe feeling only mild pressure.

Details matter more than you might expect. Evidence-based guidelines recommend using soft-tipped catheters over rigid ones, since softer catheters cause less trauma to the uterine lining and are associated with higher pregnancy rates.¹⁸ Ultrasound guidance during transfer helps the doctor place the catheter tip in the right spot, typically about 2 centimeters from the top of the uterus.¹⁹ A small air bubble is loaded into the catheter along with the medium to prevent the embryo from drifting out of position.

How Many Embryos to Transfer

Most clinics today strongly encourage transferring a single embryo at a time. Transferring two might seem like it doubles your odds, but it dramatically increases the risk of twins, and twin pregnancies carry substantially higher rates of preterm birth, low birth weight, and complications for both the parent and the babies. A large UK registry study found that transferring two embryos at once was associated with roughly 28 times the odds of multiple birth compared to single embryo transfer.²⁰ When cumulative results were tallied, two consecutive single transfers offered a comparable live-birth rate to a double transfer, but with far fewer complications. This is why “one at a time” has become the standard recommendation in most countries.

Fresh Versus Frozen Transfers

You might transfer a fresh embryo in the same cycle as your retrieval, or the lab can freeze all embryos and transfer one in a later cycle. Frozen embryo transfer (FET) has become increasingly common, partly because the hormonal environment after stimulation is not always ideal for implantation. A study comparing over 62,000 initial FETs with more than 166,000 fresh transfers found higher live-birth rates with frozen transfers across all age groups.²¹ Neonatal outcomes also differ: a meta-analysis reported that fresh transfers carried higher rates of preterm birth, low birth weight, and small-for-gestational-age babies, while frozen transfers were more likely to result in larger-than-average babies.²² Neither approach is risk-free, but the trend toward freeze-all cycles reflects the accumulating evidence that letting the uterus recover from stimulation before transfer tends to improve results.

The Endometrial Receptivity Question

A test called the endometrial receptivity array (ERA) was introduced in 2011 to identify whether your uterine lining is ready for implantation at the expected time, or whether your personal “window of implantation” is shifted earlier or later than average. The evidence on ERA is mixed. In patients with a good overall prognosis, the test does not seem to improve outcomes. But for patients who have experienced repeated implantation failure despite transferring chromosomally normal embryos, the ERA has shown more promise.²³ It is an added cost and involves an extra biopsy cycle, so it is typically reserved for when standard approaches have not worked.

Stage 5: Luteal Support and the Wait

After transfer, your body needs progesterone to thicken and maintain the uterine lining so a newly implanting embryo can survive. In a natural conception, the ovary’s corpus luteum provides this. In an IVF cycle, the stimulation drugs and egg retrieval disrupt normal corpus luteum function, so supplemental progesterone is prescribed for roughly the first 8 to 12 weeks of pregnancy if the cycle is successful.

Progesterone comes in several forms: vaginal gels or inserts, intramuscular injections, oral tablets, and subcutaneous injections. Patients often have strong feelings about these, since the intramuscular shots can be painful and the vaginal preparations can be messy. In one study of frozen-transfer cycles, vaginal progesterone gel produced higher implantation, delivery, and live-birth rates than intramuscular injections, and a lower rate of early miscarriage.²⁴ Other studies, however, have found no significant difference between the two routes in terms of live-birth rates.²⁵ A recent network meta-analysis of 24 randomized trials in fresh cycles concluded that oral and intramuscular routes may be the most effective at improving live-birth rates, while vaginal and subcutaneous routes did not reach statistical significance for that endpoint.²⁶ The bottom line is that there is no single best route for everyone, and your clinic will pick one based on your cycle type, your comfort, and the available evidence.

About 9 to 14 days after transfer, a blood test measures your beta-hCG level, the earliest reliable marker of pregnancy. If the number is positive, you will repeat the test about 48 hours later. If the hCG level doubles over that window and reaches at least 100 mIU/mL by 15 days after retrieval, the odds of a live birth are strong. In one large study, when both of those benchmarks were met, about 85% of cycles resulted in a live birth.²⁷ A slow or low initial rise, on the other hand, can signal an ectopic pregnancy or an early loss, and your clinic will monitor you closely.

Ovarian Hyperstimulation Syndrome

The main medical risk unique to IVF is ovarian hyperstimulation syndrome (OHSS), which happens when the ovaries overreact to the stimulation drugs. The ovaries enlarge, and fluid leaks from blood vessels into the abdomen, sometimes into the chest cavity as well. Mild OHSS causes bloating, nausea, and abdominal discomfort. Severe OHSS can lead to significant fluid accumulation (ascites), breathing difficulty from pleural effusion, reduced urine output, and blood clots.²⁸ Severe cases require hospitalization and, in rare instances, can be life-threatening.

OHSS is triggered by hCG, which is why using a GnRH agonist trigger instead of an hCG trigger in antagonist protocols has become a key prevention strategy. Other protective steps include using lower gonadotropin doses (especially in patients with polycystic ovary syndrome, who are at higher risk), using the antagonist protocol rather than the long agonist protocol, and freezing all embryos instead of doing a fresh transfer so that a pregnancy-related hCG surge does not worsen the condition.⁵ Baseline albumin, total protein, and hematocrit levels can help predict who is at risk for more severe disease.²⁸

The Emotional Dimension

IVF is physically demanding, but the psychological toll can be just as significant. The daily injections, frequent clinic visits, uncertainty at each stage, and the financial pressure create a sustained stress response that is easy to underestimate. A systematic review found that stress and anxiety most strongly affect the egg-retrieval stage, and that chronic stress is particularly associated with fertilization, embryo transfer, and pregnancy outcomes.²⁹ This does not mean stress “causes” IVF to fail. Rather, the emotional and hormonal burden is real, and patients dealing with infertility experience a loss of control and life-planning certainty that compounds with every cycle.³⁰

Many clinics now screen for anxiety and depression at intake and between cycles. If yours does not, raising the subject yourself is worthwhile. Counseling, peer support groups, and mind-body interventions like cognitive behavioral therapy have all shown benefits in helping people cope with the demands of treatment. Partners are affected too, even though the medical procedures fall on one person’s body. Treating IVF as a shared emotional experience, not just a medical one, tends to make the process more manageable for everyone involved.