Each menstrual cycle, a woman naturally develops and releases only one mature egg. If this single egg were retrieved for in vitro fertilization (IVF), it would result in only one possible embryo, significantly limiting the chances of conception.  

In the early 1980s, the first IVF procedures were performed in natural cycles, meaning no ovarian stimulation was used. However, success rates were low. The introduction of ovarian stimulation protocols allowed doctors to obtain multiple mature eggs per cycle, increasing the chances of creating several embryos and boosting pregnancy success rates for timed intercourse, intrauterine insemination (IUI), or IVF.  

That said, some fertility centers still offer natural-cycle IVF in select cases, such as young women with severely diminished ovarian reserve or those who cannot undergo ovarian stimulation due to hormone-sensitive conditions like breast cancer.  

 Are All Ovarian Stimulation Protocols the Same?  

There are two main types of ovarian stimulation:  

1. Stimulation for Natural Conception or IUI  

   - The goal is to limit the number of mature eggs, preventing the risk of high-order multiple pregnancies (triplets, quadruplets, etc.).  

   - Since fertilization occurs naturally inside the body, multiple eggs could be fertilized at the same time by millions of sperm, leading to uncontrolled multiple gestations.  

   - For this reason, ovarian stimulation in these cases is mild, typically aiming for only one or two eggs to mature.  

2. Stimulation for IVF  

   - Here, fertilization happens in a laboratory, so doctors can control how many embryos are transferred back into the uterus.  

   - Aggressive ovarian stimulation is used to retrieve as many mature eggs as possible in one cycle, maximizing the chances of obtaining viable embryos.  

   - Any extra embryos can be frozen for future attempts, allowing for carefully controlled embryo transfers, significantly reducing the risk of multiple pregnancies.  

By transferring a single embryo, the risk of twins or higher-order multiples is greatly minimized. However, in rare cases, a single embryo may split after transfer, leading to identical twins—a phenomenon that is unpredictable and naturally occurring.