On this page you will find information about pregnancy loss, including; the New Zealand definition of miscarriage; the prevalence of miscarriage; the most common cause of miscarriage; and recurrent miscarriage and its causes.
Definition of miscarriage in New Zealand
A miscarriage is a pregnancy that ends spontaneously before 20 weeks gestation. After 20 weeks, or if the birth weight of the baby is over 400gm, the end of a pregnancy is classified as a birth; either the birth of a live baby or a still birth (1).
How common is miscarriage?
Please note: The statistics provided on this page are sourced from the most robust and relevant studies that we could find. We choose not to include unknown miscarriages when talking about miscarriage rates. We also draw distinction between the prevalence of miscarriage per woman and the prevalence of miscarriage per pregnancy.
Prevalence of miscarriage per woman
A study in 2008 found that, of New Zealand women who had been pregnant at some point in their lives, almost one third of them reported that they had experienced at least one miscarriage (2). This may be surprising if you are feeling alone and blindsided by a miscarriage. Our cultural practice of waiting until after 12 weeks gestation before announcing a pregnancy means that we can be quite unaware of the prevalence until we experience it. You are not alone.
It can make us feel better to know that miscarriage is reasonably common, and that it is most often a completely natural and spontaneous occurrence. But common, natural occurrences can still be life altering; your experience matters and it is unique to you.
This information is based on data from Fanslow et al., 2008, and relates to the prevalence of miscarriage as reported by 2391 New Zealand women who had all experienced pregnancy at some time in their life (2).
Prevalence of miscarriage per pregnancy
The prevalence of miscarriage in pregnancy is strongly influenced by maternal age (the age of the pregnant mother). According to a 2019 study (3), 25 - 29 year olds are at the lowest risk for miscarriage (10% risk). Surprisingly, mothers under 20 years of age were found to have a similar risk of miscarriage as mothers aged 35-39 (around 16%). Risk of miscarriage gets significantly higher if maternal age is over 40 years; by 40-44 years old a mother’s risk of miscarriage rises to 32%, and women over 45 were found to have a 53% chance of a miscarriage.
Risk of miscarriage in first pregnancy in relation to maternal age
This scale is based on approximate data from Magnus et al., 2019 - a prospective register based study of Norwegian women who were pregnant between 2009 and 2013 (421,201 pregnancies).
NOTE: ‘miscarriage per woman’ and ‘miscarriage per pregnancy’ are different statistics.
For example, if 100 women had two children each and a total of 33 miscarriages, between them that would be 233 pregnancies all together. One third of the women could have experienced a miscarriage (33/100 women) but only 14% of the pregnancies would have resulted in miscarriage (33/233 pregnancies). The positive thing to take away from this is that, although there are lots of people out there who experienced miscarriage, most of them will also experience pregnancies in which miscarriage does not occur.
What causes miscarriage?
It is important to remember that in the vast majority of cases, nothing could have been done to prevent a pregnancy being lost. Also, there is no intervention that can save a baby once a miscarriage is inevitable. New Zealand is one of the best places in the world for premature babies to “survive and thrive,”(4) but even here, babies born between the gestations of 23 weeks+0days and 24 weeks+6days are on the borderline for whether or not medical intervention would be able to help them - this is known as ‘periviable gestation’ or the cusp of viability for active medical treatment (5). It is very unlikely that modern medicine could do anything to save a baby born before 23 weeks.
The most common cause of miscarriage, accounting for about 70% of cases, (6) is a random genetic abnormality, or ‘chromosomal abnormality’.
Why are chromosomal abnormalities so common?
Every person is made up of trillions of cells, and each cell carries a full copy of that person's genetic code, in the form of chromosomes (7). During the normal fertilisation of an egg, 23 chromosomes from the father’s sperm and 23 chromosomes from the mother’s egg join to make a cell. After approximately 30 hours the single cell will divide for the first time, making two cells. Three days after fertilisation the egg will contain 16 cells, by nine days it will contain hundreds of cells (8). That is a lot of copying of chromosomes, and errors can occur at any stage; including when the mother and father each make their original 23 chromosomes. Chromosomes can end up in the wrong cell or get lost completely, resulting in a cell that has an uneven number of chromosomes - this is known as aneuploidy. 90% of genetically abnormal pregnancies will result in a miscarriage in the first trimester. Trisomy 21, three copies of chromosome 21, the genetic cause of Down syndrome, is one of the rare forms of aneuploidy in which the baby can survive to live birth (9).
What testing will be available?
For most families who experience a miscarriage the cause remains unidentified. Studies show that after three miscarriages, being supported in a dedicated clinic can help with healthy pregnancy outcomes, whereas after one or two miscarriages, healthy future pregnancies are common, without needing a specialist clinic (10). It is most likely that no testing for the cause of a miscarriage will be offered unless, as a couple, you have experienced three consecutive miscarriages (see recurrent miscarriage). However, you may be offered tests after two early miscarriages if you are in your late 30s or early 40s, or if it has taken you a long time to conceive. Talk to your health professional.
It is natural to want answers when a miscarriage occurs, so it can feel quite unsatisfactory to be offered no avenue for investigation prior to being classed as experiencing recurrent miscarriage. This is simply because it is very unlikely that a cause will be found. Of those couples who are referred to a specialist, 50% will still come away with no explanation for their recurrent pregnancy losses (6). Even though they are everywhere, babies are miracles and there is still so much to learn about how the whole process works and what causes it to go wrong. A comforting thing to take from all of this is that not finding anything wrong could mean that a healthy pregnancy could be achieved in the future with no medical intervention. If you are interested in other possible causes please see, known causes of recurrent miscarriage. There may be some tests that you could request directly from your GP.
What is recurrent miscarriage?
Recurrent miscarriage, or recurrent pregnancy loss (RPL), is when a couple suffers three or more miscarriages in a row. As many as one in 20 couples experience two miscarriages in a row, and one in 100 experience three or more in a row (3)(11). Before the occurrence of three consecutive miscarriages there is still a high likelihood that it was simply unlucky for miscarriage to have occurred; including multiple non-consecutive miscarriages, two consecutive miscarriages, or consecutive miscarriages with different partners. Only once three miscarriages of clinically recognised pregnancies have been experienced in a row, with the same partner, is there a higher possibility that some other cause may be at play.
Once classified as experiencing recurrent miscarriage there is a higher chance that a treatable cause could be at play. Here are some possible causes for recurrent miscarriage:
found in 3-5% of couples with recurrent miscarriage.
congenital uterine anomalies such as a septate uterus, diethylstilbestrol-linked anomalies or acquired uterine anomalies such as intrauterine adhesions, fibroids, polyps, or incompetent cervix.
Blood clotting disorders
conditions such as factor V Leiden and other blood clotting conditions could play a part in recurrent miscarriage.
abnormal maternal immune response to pregnancy. This is a controversial area which has yet to be fully understood or consistently and reliably diagnosed. The Antiphospholipid syndrome is the only proven autoimmune disease associated with recurrent miscarriage.
Issues with endocrine glands
uncontrolled diabetes or thyroid disease, possible association with polycystic ovarian syndrome or Hyperprolactinaemia (the presence of abnormally high levels of prolactin in the blood)
advancing maternal or paternal age, weight (under or overweight), smoking, alcohol and recreational drugs.
If you would like to know more about recurrent miscarriage, its causes and treatment, check out this webinar hosted by Fertility Week with New Zealand doctor, Karen Buckingham (14).