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Female Body

How common is miscarriage and why does it happen?

Elizabeth Oliver

What is a miscarriage?

Miscarriage is the spontaneous loss of a pregnancy before 20 weeks of gestation (1). It can also be referred to as a spontaneous abortion (as opposed to a medically induced abortion). After 20 weeks of pregnancy, the term stillborn is used. Miscarriage is a common complication of early pregnancy but can be emotionally challenging. There are many misconceptions surrounding the cause of miscarriage and because of this women often blame themselves. Understanding how common miscarriage is and why it happens is essential to reducing the stigma that many women face after experiencing pregnancy loss.

How common are miscarriages?

Miscarriage occurs in around 15% of clinically detected pregnancies, which is a pregnancy confirmed by ultrasound (2, 4). Maternal age impacts the rate of miscarriage (we will discuss the reason for this later), with the risk of miscarriage increasing from approximately 9% in women aged 20 - 24 to around 75% in women aged 45 and over (3).

Miscarriage can also happen after implantation but before a pregnancy is detected clinically. These undetected pregnancies can often be mistaken for a late period and can make estimating the actual rate of miscarriage difficult. When taking undetected pregnancies into account, the miscarriage rate is expected to be closer to 1 in 3 (2, 4).

What are the signs of a miscarriage?

The main sign of miscarriage is vaginal bleeding. This is usually accompanied by cramping and pain in the lower abdomen. It is important to remember that light vaginal bleeding - often known as spotting - is also a common symptom of early pregnancy. Vaginal bleeding is thought to affect about 1 in 4 people and is usually nothing to worry about (5). If you are pregnant and experience heavy spotting or bleeding you should always contact your healthcare provider.

What are the types of miscarriage?

There are several different types of miscarriage:

Complete miscarriage - all the pregnancy tissue is expelled from the body.

Threatened miscarriage - there is vaginal bleeding, but the cervix has not begun to dilate.

Inevitable miscarriage - the cervix is open and there is heavy bleeding and abdominal cramping. In this case, miscarriage cannot be avoided.

Incomplete miscarriage - most of the pregnancy tissue has been expelled, but some remains in the uterus.

Missed (or silent) miscarriage - the embryo has stopped developing but instead of being expelled from the body remains in the uterus. 

Other types of pregnancy loss

Other types of pregnancy loss include:

Chemical pregnancy - an early pregnancy loss that happens shortly after implantation (usually before 6 weeks of gestation). Chemical pregnancies take place before the pregnancy can be detected by ultrasound but can be detected by testing for hCG. hCG is the hormone produced by the placenta from the time of implantation.

Molar pregnancy (also known as hydatidiform mole) is a rare complication of pregnancy characterised by the abnormal growth of trophoblasts which are the cells that normally develop into the placenta. A molar pregnancy can have serious complications and requires early treatment.

Blighted ovum (also known as an anembryonic pregnancy) is a fertilised egg which implants into the uterus forming a gestational sac but fails to develop. On ultrasound it is detected as an empty gestational sac.

Ectopic pregnancy occurs when a fertilised egg implants outside the uterus, usually in the Fallopian tube. The fertilised egg cannot survive and growth of the pregnancy can cause the tube to rupture leading to internal bleeding. If you suspect you have an ectopic pregnancy you must seek immediate medical help.

What are the reasons for miscarriage?

Chromosomal abnormalities

Most miscarriages happen in the first 12 weeks of pregnancy (2). The majority of these early miscarriages (over 50%) are caused by abnormalities in the number of chromosomes present in the embryo (6). Chromosomes contain our genetic material, and each cell normally has 46 grouped in 23 pairs. During fertilisation, each parent contributes 23 chromosomes to make a total of 46. Abnormalities occur if the sperm or egg that have fused to form the embryo do not have the correct number of chromosomes. Chromosomal abnormalities can be genetically inherited, but more often than not occur spontaneously during cell division of the egg or sperm or during the development of the embryo. Early pregnancy losses are simply a mechanism to prevent embryos that are not developing properly from continuing further development and are a normal part of the reproductive process. The risk of spontaneous chromosomal abnormalities increases with maternal age (7). A recent study has found evidence to suggest that paternal age may also have an impact on the risk of miscarriage in men aged 40 years or over (8).

Other factors

Certain hormonal conditions such as thyroid dysfunction and polycystic ovary syndrome (PCOS) can increase the risk of miscarriage (9, 10). Another contributory factor can be structural problems with the uterus (11). The risk of miscarriage can also be increased by drinking alcohol, as well as extremes in maternal weight (both obesity or being underweight) and high levels of stress (12, 13, 14).

Do I need medical treatment for a miscarriage?

Most early miscarriages do not require medical treatment and can be managed by a “watch and wait” approach, sometimes referred to as expectant management. With this option, over 70% of women will pass the pregnancy tissue within two weeks (15). Intervention may be necessary if there is a risk of complication or if there is remaining pregnancy tissue in the uterus. Some people may also opt for intervention to have more control over the situation, or if they want it to be over more quickly. Interventions can include taking a medication called misoprostol. Misoprostol triggers the body to expel the tissue by causing the cervix to open and the uterus to contract. The process will usually start within hours of taking the pill. If the tissue does not pass naturally or with medication, a small surgical procedure called dilatation and curettage (D&C) can be performed. After dilating your cervix, your doctor will use a spoon-shaped object called a curette to remove tissue from the inner lining of your uterus.

Progesterone and pregnancy

Progesterone is a hormone released by the corpus luteum during the second half of the menstrual cycle (also known as the luteal phase) (16). It is necessary to prepare the endometrium for the implantation of the fertilised egg, and to support the early stages of pregnancy. Luteal phase deficiency (LPD) is a condition where the production of progesterone is below normal, or when the endometrium does not respond to the normal stimulation of progesterone (17). As a result, the endometrium is unable to properly prepare for the implantation of the fertilised egg. LPD has been associated with disorders such as PCOS and hypothyroidism and has been linked to difficulties getting pregnant and miscarriage (18). There is still some uncertainty surrounding the cause and diagnosis of LPD. One of the signs can be a luteal phase that is shorter than normal (17, 18). Although progesterone levels can be low, even if the luteal phase is a normal length (19). While inne cannot be used for any type of diagnosis, tracking your progesterone levels with inne can help you identify any irregularities in your progesterone profile or the length of your luteal phase. Any concerns you may have should be discussed with your healthcare provider.

How soon can you get pregnant after a miscarriage?

It can take several weeks for the blood and tissue from a miscarriage to completely expel from the uterus. If a miscarriage happens within the first 12 weeks of pregnancy, menstruation can return within 4-6 weeks (20). This means it is possible to be fertile again immediately after a miscarriage.

It is normal to feel apprehensive about trying to conceive again following a miscarriage. You should ensure that you feel both emotionally and physically ready before trying again.


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