Miscarriage Risk by Week

When you lose a pregnancy before 20 week’s gestation, this is called a “miscarriage”.  Most of the time, there is no explanation for the pregnancy loss, although some women have health conditions that cause them to have repetitive miscarriages.  In general, only about 10-20 percent of women will have a miscarriage although risk of miscarriage varies according to the week.

Miscarriages usually happen in the first through thirteenth week of pregnancy.  Many of them occur before the woman even knows she is pregnant or shortly after having a positive pregnancy test.   Many miscarriages occur because the fertilized egg did not implant into the uterine wall and there may not even be a positive pregnancy test in the first place.  Instead, the woman will have bleeding that is just a couple of days after she expected to have her period.

Regardless of when you have a miscarriage, those that occur before the   13^th-20^th week are generally due to a chromosome problem with the baby.  Either the sperm or the egg was unhealthy or damaged and the baby was conceived having abnormalities in the chromosomes.  There can also be a problem with the division of cells in the zygote phase of the pregnancy.

Other causes of miscarriage include the following:

• The egg doesn’t implant properly in the uterus
• There is an infection of the uterus
• There is a hormonal problem in the mother
• The mother is of an advanced age
• The mother had a poor lifestyle, such as smoking cigarettes, drinking too much caffeine, drug exposure or exposure to chemicals in pregnancy
• Some type of traumatic injury in the mother

When considering all pregnancies, the miscarriage rate is about 20 percent per pregnancy in the first twelve weeks of the pregnancy.  Here is the average rate of pregnancy loss per week:

• Week 1 to 2: The miscarriage risk is about 75 percent
• Week 3 to 6: The miscarriage rate is 10 percent
• Week 6 to 12: The miscarriage rate is 5 percent
• Week 12 to 20: The miscarriage rate is about 3 percent

Your chances of having a miscarriage depends on how many miscarriages you have had in the past.  For example, if you had a single miscarriage in the past, there is a 10-13 percent chance of a miscarriage in the future.   If you have had one live birth and one miscarriage, the rate of miscarriage is 10 percent. If you have had two pregnancies that both ended in miscarriage, your risk of miscarriage is 40 percent.  If you have had at least one live birth, your risk of miscarriage is 13 percent. If you have had three pregnancies ending in miscarriage, your miscarriage rate is 60 percent. If you have had no live births and had four miscarriages, you have a 95-100 percent chance of having another miscarriage.  It usually means that you have a health condition that is causing repetitive miscarriages.

The Risk of Recurrent Miscarriages

If you have just had a single miscarriage, you have no greater risk for a miscarriage in the future.  If you have had more than one miscarriage in the first trimester or a pregnancy that was lost between 12 and 20 weeks, you should see your doctor to see if there is a treatable condition that can prevent another miscarriage in the future.

There are a few treatable conditions you can have that can be treated early in the pregnancy to lessen the chance of a miscarriage.  These include thrombophilia, which is blood clots in pregnancy, diabetes, thyroid disease, and autoimmune disorders.  These will increase your chances of having a miscarriage but are treatable so you may be able to have a viable pregnancy.

The Effect of Age on Miscarriage Rate

As a woman ages, the chances of having a miscarriage goes up.  This is the general rule for miscarriages and age:

• Less than 35 years of age—the risk of miscarriage is 15 percent
• 35 to 45 years of age—the risk of miscarriage is between 20 and 35 percent
• Older than 45 years of age—the risk of miscarriage is 50 percent

Older mothers have a better chance of carrying a pregnancy the entire way through if she is treated with hormone replacement therapy in the early stages of your pregnancy.  If there is a chromosome problem, you can have genetic testing to make sure you don’t have a genetic disorder.