To answer this question we will have to understand what happens during the first few weeks of pregnancy. When the egg released by the follicle in your ovary meets the sperm - Fertilisation happens and it leads to pregnancy. Every month during the first part of the menstrual cycle, many follicles containing the maturing egg begin to develop. Only one of the follicle matures and during ovulation, the follicle ruptures and the egg is released.

After the release of the egg, the remaining part of that follicle begins to produce progesterone which helps to thicken and prepare the uterine lining for implantation of the fertilized egg. The corpus luteum continues to produce progesterone for the next 12 to 16 days. Around 10 weeks of pregnancy the placenta would have developed and it begins to produce progesterone, thus taking over the responsibility of maintaining and developing the pregnancy form corpus luteum.

Progesterone is the main hormone which is needed to maintain a healthy pregnancy until the birth of the baby. Progesterone does this job by inhibiting the immune response of the body (otherwise the sperm can be construed as a foreign substance and the mother’s body can launch an immune reaction against both the sperm and the developing zygote). Progesterone also decreases prostaglandins and inhibits uterine contractions. It is for this reason that if the woman does not produce adequate progesterone during the early part of pregnancy, it can lead to pregnancy loss.

Progesterone levels also can have quite a variance at this stage of pregnancy. They can range from 9-47ng/ml in the first trimester, with an average of 12-20 ng/ml in the first 5-6 weeks of pregnancy.

In situations when there is a concern about an ectopic pregnancy or miscarriage, progesterone levels will be low from the beginning.

Between 5 ½ to 6 ½ weeks, a fetal pole or even a fetal heartbeat may be detected by vaginal ultrasound. If a vaginal ultrasound is done and no fetal pole or cardiac activity is seen, another ultrasound scan should be done in 3-7 days. Due to the fact that pregnancy dating can be wrong, it would be much too early at this point to make a clear diagnosis of the outcome of the pregnancy.

So when the doctor is concerned about the pregnancy and suspects low levels of progesterone, he /she may suggest you progesterone tablets, sometimes till the third trimester.