This is actually a very easy question to answer, because it is always the same answer. To best explain, a biology lesson is needed. Each month, when a woman ovulates, the follicle where the egg is expelled creates a small cyst. This cyst secretes the hormone progesterone to support the pregnancy (if it occurs) until the placenta is large enough to take over around 7 to 8 weeks. In surrogacy, we suppress the Surrogate's natural cycle and she does not ovulate. Since she does not ovulate, there is no cyst to produce progesterone. That is why the supplement is needed to sustain the pregnancy until it is far enough along for the placenta to safely support it. Most clinics supplement until 10 to 12 weeks of pregnancy to be on the safe side. Every clinic has a different protocol, they all use different medications, different doses and different calendars. That being said, they all have the same goal. Most protocols involve birth control pills, Lupron, estrogen and progesterone. There are different ways to administer most of the hormones and it is important to work with the clinic to find the best option for each situation.
Intended Parents always ask why the hormones are needed, but the Surrogates always ask how they are administered. The reason behind this question is because most women dislike needles. It is almost impossible to do a medicated cycle without utilizing injections at some point, but there are ways to minimize how many are needed. Progesterone in oil is the biggest of the needles, it is a large intramuscular injection given once or twice a day in the hip/butt. There are alternative to using PIO, there are vaginal suppositories and gels. There is no needle involved with these options, but they tend to be messy. Lupron is a daily injection giving with a small subcutaneous needle. The injection is not very painful or intimidating.
The last important thing to note about hormones, is how they will make your surrogate feel. It goes with out explanation that the medications will alter her mood. She may feel sad, weepy, tired, hungry, moody, and angry. Some women have very little side effects from the therapy and a few have severe side effects, but most fall somewhere in the middle. The good news is that the effects are short lived (until the pregnancy hormones take over) and for a good cause. It is important to be aware that your surrogate may be more emotional during cycling. Acknowledgement of her feelings is very comforting during this stage of the process.