How Progesterone Levels Affect Early Pregnancy

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Progesterone levels rise during early pregnancy to support implantation and development of your embryo and fetus. A low progesterone level during the early weeks increases your chance of a poor pregnancy outcome in the first trimester.

Pregnancy and Progesterone Levels

In a pregnancy cycle, your progesterone level continues to rise in the days following fertilization of your egg and implantation of your embryo. It is essential for your baby's early development.

Levels Throughout Pregnancy

During the first few weeks of your pregnancy, the increasing levels of progesterone come from the corpus luteum in your ovary. After about 10 to 12 weeks, the placenta becomes the main source of the hormone, which has important functions throughout pregnancy.

According to the United States National Library of Medicine, normal progesterone levels range throughout pregnancy as follows:

  • First trimester: 11.2 to 90 ng/ml
  • Second trimester: 25.6 to 89.4 ng/ml
  • Third trimester: 48 to 150 to over 300ng/ml

Measuring Progesterone

Progesterone is usually measured in the first few weeks of pregnancy if there is a concern about the viability of the embryo or fetus. A low progesterone level may indicate a nonviable pregnancy.

Less commonly, levels are higher than expected, which may be due to:

Your doctor will investigate a high level further with other blood tests, an ultrasound, or other imaging studies.

Levels In Normal and Abnormal Pregnancies

In general, though levels overlap, progesterone is higher in early normal intrauterine than in abnormal pregnancies. According to the classic textbook, Clinical Gynecologic Endocrinology and Infertility (pages 1393-1394), in the first four to six weeks of pregnancy if your progesterone level is:

  • Higher than 20 nanogram/milliliter (ng/ml): You have almost a 100% chance of having a viable pregnancy in your uterus.
  • Lower than 5 ng/ml: You have a high risk that you have a nonviable pregnancy (intrauterine embryo or fetus or an ectopic pregnancy).
  • Between 5 and 20 ng/ml: There is 70% chance that you have a viable intrauterine pregnancy.

A low progesterone level is not a good predictor of an ectopic pregnancy, however, according to the textbook.

Evaluation of Low Progesterone in Early Pregnancy

You may have concerns about low progesterone during pregnancy. Because there is an overlap in progesterone levels in early viable and nonviable pregnancies, if your progesterone is low, your doctor will combine your result with the following tests to help evaluate your pregnancy and where your fetus is located:

  • Serial BHCG levels: According to Clinical Gynecologic Endocrinology and Infertility (page 1389), beta human chorionic gonadotrophin level (BHCG) doubles about every 48 hours during the first six weeks of pregnancy. A low initial BHCG, or a non-doubling or falling serial BHCG, with low progesterone makes it more likely you have a nonviable pregnancy.
  • Ultrasound: A transvaginal ultrasound can help determine if there is a pregnancy in your uterus, which rules out ectopic implantation. According Berek and Novak's Gynecology (page 613 to 615), an intrauterine sac is visible by transvaginal ultrasound (U/S) at about four to five weeks of pregnancy and when the BHCG is 1,500 milliInternational Units per milliliter (mIU/ml). By transabdominal U/S, a sac is visible when the BHCG is 6,500. At six weeks and after, an early fetus with heart activity confirms a viable pregnancy

Your previous fertility and pregnancy history, as well as your current history and symptoms - such as vaginal bleeding or pelvic pain - will help determine your evaluation and diagnosis.

Causes of Low Progesterone Levels in Early Pregnancy

The most common causes of low progesterone levels in early pregnancy include:

  • Ectopic pregnancy, where the pregnancy implants in a fallopian tube, an ovary, the cervix, or in the pelvic or abdominal cavity. An ectopic pregnancy is the most important cause to look for because it can become life-threatening if not diagnosed early.
  • An early embryo or fetal loss because of chromosomal abnormalities
  • A complete or incomplete miscarriage, which occurs in 10 to 25% of pregnancies
  • An inevitable or threatened miscarriage, which may or may not continue on
  • A chemical pregnancy where the embryo doesn't develop beyond four weeks. This will be associated with a low BHCG
  • Early death of the corpus luteum

These abnormal pregnancies may or may not be associated with vaginal bleeding or pelvic and abdominal cramps or pain.

Treating Low Progesterone in Early Pregnancy

When the progesterone level is low, and there is a viable intrauterine pregnancy on ultrasound, the diagnosis might be a threatened miscarriage. The problem often causes vaginal spotting or bleeding and pelvic cramps.

Some doctors prescribe progesterone to prevent a miscarriage in the case of a low progesterone level. However, treatment of threatened miscarriage with progesterone is controversial, according to research evidence.

An Analysis of the Research

An analysis of studies in the Cochrane Library database on the use of progesterone to prevent miscarriages was first published in 2008 and updated in 2013. Based on these studies, the authors concluded there is no evidence treatment with progesterone supplements by intramuscular injections, vaginal suppositories, or oral pills during early or mid pregnancy prevents miscarriages. According to the study, progesterone therapy may be appropriate, however, if you have a history of recurrent miscarriages.

The Role of Progesterone in Early Pregnancy

In addition to anchoring the embryo in the endometrium and nurturing its development to a fetus, the role of progesterone in early pregnancy includes the following:

  • Relaxes the muscle layer of the uterine wall for the growth of the fetus and to reduce the chance of contractions
  • Forms the mucus plug, which will last until the end of the third trimester of pregnancy
  • Protects the baby from rejection by the mother from her antibodies and other immune factors
  • Increases blood circulation between the uterus and placenta
  • Starts growth of the breast glands to produce milk

Talk With Your Doctor

Ask your doctor for more information and the appropriate evaluation for your circumstances if your progesterone level is low during your early pregnancy. This is even more important if you have vaginal bleeding or pelvic or abdominal pain to ensure you don't have an ectopic pregnancy.

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