Answer:

Introduction:
The scientist is conducting research to identify a hormone in the blood that can serve as a marker for pregnancy. This implies that the hormone should be present specifically during pregnancy and absent during other times. Among the given options, the hormone that will not be ideal for this purpose is progesterone.

Reasons:
1. Progesterone: Progesterone is a hormone that plays a crucial role in preparing the uterus for implantation and maintaining pregnancy. Its levels rise significantly during pregnancy and are primarily produced by the corpus luteum in the ovaries during the initial stages. However, progesterone levels can also increase during the luteal phase of the menstrual cycle, making it less specific to pregnancy. Therefore, progesterone is not an ideal hormone to serve as a marker for pregnancy.

2. Chorionic Gonadotrophin: Chorionic gonadotrophin (hCG) is a hormone that is exclusively produced during pregnancy. It is secreted by the placenta and acts to maintain the corpus luteum, which in turn produces progesterone. hCG levels rise rapidly after implantation and continue to increase throughout the first trimester of pregnancy. Due to its specificity to pregnancy and its early appearance in the blood, hCG is commonly used as a marker for pregnancy.

3. Relaxin: Relaxin is a hormone that is primarily produced by the corpus luteum during pregnancy. It helps to relax and soften the ligaments and joints in preparation for childbirth. While relaxin levels are elevated during pregnancy, they can also increase during other conditions such as ovarian cysts or certain tumors. Although relaxin can be used as an indicator of pregnancy, its specificity is lower compared to hCG.

4. Placental Lactogen: Placental lactogen, also known as human chorionic somatomammotropin (hCS), is a hormone produced by the placenta during pregnancy. It promotes breast development and stimulates milk production. Placental lactogen levels increase throughout pregnancy and are specific to gestation. Therefore, it can be considered as a reliable marker for pregnancy.

Conclusion:
Among the given options, progesterone is not an ideal hormone to serve as a marker for pregnancy due to its increased levels during the luteal phase of the menstrual cycle. On the other hand, hormones such as hCG, relaxin, and placental lactogen are more specific to pregnancy and can be used as reliable markers.