Why Miscarriages Happen

Miscarriage is a very common event

When given time for your body to recover and if you (and your partner’s) age is not against you, your next pregnancy will normally go well. (see ‘Subsequent Pregnancies’) The stats are little different from a first loss. You may find you are more vigilant and aware of what could cause problems for the next pregnancy, but that is no bad thing, unless it becomes obsessive. It is only self-protection if we do not feel the same joy of this new life as we did with our first pregnancy. We say, ‘our innocence was lost.’ Check ‘Recurrent Miscarriages’ if you experience further losses.

Although they are not talked about much, the majority of miscarriages are often the first pregnancy and usually in the first trimester (up to 14 weeks).  They are caused by a genetic problem with that particular foetus and are due to nothing more than unfortunate bad luck and therefore are unlikely to reoccur. The generally accepted figures are that between 15% – 25% of pregnancies end in miscarriage.

From our own experience we suggest 1 in 4. However, that figure could be higher according to the English Professor, Sir Robert Winston, if unreported miscarriages are included. These would usually be ones that occur at home with no obvious adverse physical after affects (we assume help from a medical professional would be requested if there was and recorded by them). Very early miscarriages only recognised when the pregnancy had been previously confirmed with a home test kit would also be included in the unreported numbers. Without the test these pregnancies are often not identified by women as they can just be seen as late, heavy periods. They are called ‘Chemical Pregnancies’. It is always advisable to have a check-up after any miscarriage, even belatedly, as a low grade infection can possibly affect future pregnancies.

Second trimester miscarriages

The reasons for these can be different from first trimester miscarriages. You may find what that reason is after tests are done, although not necessarily. However if you do and it is something repairable or treatable, this knowledge can work for you in a subsequent pregnancy.

Possible reasons for miscarriage

The following possible reasons are a collation of internet findings from medical sites recognised for their status and integrity. Several apply to recurrent miscarriage only. Some causes are generally accepted and others are still in the research stages. They have been edited by a medical professional.

The baby

  • Chromosomal Abnormalities; this is a non-viable pregnancy and the most common reason given for all miscarriages (50% to 70% quoted) and are also usually first time pregnancies. There is/was nothing that could have been done to save your baby. Nothing you did or didn’t do, like bending, stretching, carrying a heavy weight, walking, swimming, usual daily activities or having sex would have made any difference, otherwise there would be many more miscarriages. (Moderate exercise is always encouraged during pregnancy.) A baby’s remains can be tested even if the miscarriage happened at home although persistence may be required with medical professionals. If post mortem tests are carried out, usually with recurrent miscarriage, they can reveal the sex of the baby but perhaps no cause. This may not mean there is no problem. It might just be that one cannot be detected with the current medical knowledge.
  • The chromosomes are tiny thread-like structures found in the nucleus of a cell. There are 46 chromosomes arranged into 23 pairs. One chromosome per pair coming from the father and the mother. Each carries hundreds to thousands of genes which are the basic units of heredity. They are responsible for growth and development, determining all of a person’s physical attributes, such as sex, hair, eye colour and blood type to the workings of internal organs. At conception, a cross-over of genes takes place with half of them coming from the father’s sperm and the other from the mother’s ovulating egg. This forms a baby. At this critical time the cell may split unevenly and information, lost by chance, will lead to a miscarriage.
    The information may not be required for many weeks so a pregnancy can appear normal initially until the missing piece is needed, then, with-out it; inevitably a baby dies and miscarries. Abnormalities involving a missing or extra chromosome are not caused by an exposure during pregnancy. Random chromosomal abnormalities are unlikely to reoccur and are not the result of either parent’s health problems. Once a pregnancy is affected by a chromosome abnormality though, there is however a slightly greater chance for future pregnancies to be affected also. In some cases prenatal diagnosis are offered for future pregnancies such as chorionic villus sampling (CVS) or amniocentesis.
  • Inherited Chromosomal Rearrangements; only 5% of couples have these chromosomal abnormalities which are passed on to their babies and they can come from either parent. They can cause a wide range of defects. When recurrent miscarriages occur there is approximately a 2-4% chance that a baby has inherited an extra chromosome or is missing a piece of one, but healthy children can still be conceived in the future. Chromosome studies can be performed on parents’ blood to see if either is a carrier of a chromosomal re-arrangement. Parents themselves should have no health problems because, although their chromosomes are rearranged, they are balanced and there are no missing or extra pieces of the chromosomes.
  • Gene Mutation; another genetic cause is a change (mutation) in a single gene (or several genes) on the chromosomes. This can cause specific genetic diseases or birth defects. Mutations can occur spontaneously or can be inherited from parents who themselves are healthy. Nature often detects a baby’s incompatibility to life and the foetus is passed naturally. It is a good idea to let your LMC (Lead Maternity Carer) know early on about any current or historical family members with genetic problems. A geneticist could be consulted.
  • Egg implantation; the fertilised egg may implant outside the uterus (womb – most often within a fallopian tube) resulting in an Ectopic pregnancy. This is a life-threatening situation that is often agonising and distressing.
  • The placenta (afterbirth); could be imperfect and not functioning properly contributing to a poorly nourished and underweight baby.

The Mother may

  • Be sick or under too much stress, the effect depending on to what degree with each individual woman
  • Have serious morning sickness (hypermesis gravidarum) which causes dehydration requiring medical attention (see ‘Things we may not know’ & ‘Morning Sickness‘)
  • Still have an intrauterine contraceptive device (IUCD) inside the uterus and 99.9% will cause the contents of the uterus to abort
  • Have maternal hypothyroidism. https://hypothyroidmom.com/thyroid-hormone-birth-defects-and-the-developing-brain. We also recommend women get their thyroid panels checked, (including antibodies) before conceiving.
  • Have suffered serious stress or injury and although even a hard blow to the stomach is survivable, it can cause placental abruption and loss of baby. Injuries to other parts of the body may cause other problems affecting the baby’s survival.
  • Have hormone problems within her reproductive system like Luteal Phase Defect – hormone imbalance with menstrual phases out of kilter. PCOS and Estrogen dominance may also contribute to miscarriage.
  • Have gluten intolerance (blood test diagnosis) which has very specific symptoms which a GP and nutritionist can control with diet when diagnosed (Coeliac Disease)
  • Have ‘NK’ (Natural Killer) Cells. These are the ones responsible for protecting us from bacteria, viruses and foreign bodies, and organ transplants rejections. The developing baby contains foreign genetic material coming from the father, but in normal circumstances it is not rejected. However, in some women these NK cells may do so and cause a miscarriage either by being high in numbers or by abnormal hostile activity. Like most auto-immune disorders this problem can switch on and off, so some of the women may have 1 or more normal pregnancies as well as recurrent miscarriages. Women with CD16/56 NK cells in excess of 12% or 0.2 absolute number are at risk but the current research is very hopeful.
  • Have small fibroids usually after a previous caesarean operation
  • Are histamine intolerant which occurs when the capacity to degrade histamine is reduced. It causes all the smooth muscles of the body to contract, this includes the uterus which makes it difficult for the egg to adhere to the uterine wall.  https://www.lapislight.com/wp/tag/miscarriage/
  • Have a physical defect like uterine malformation (one in 6 to 10 women) or Cervical Incompetence which only affects pregnancies beyond 14 weeks gestation. The insertion of a cervical stitch (cerclage procedure) is often recommended and some doctors will also prescribe bed rest. The stitch requires removal 3 weeks before due date. There is no reliable method to pre-diagnose this condition which can be the result of a congenital weakness in the cervix, cervical trauma (such as from D&C procedures), multiple births or other complications during labour or delivery. The end result is the same – the cervix opens before it should.
  • Have adhesions inside the womb cavity (Asherman’s Syndrome) that can be associated with miscarriage. Possibly caused by restricting the implantation and growth of the foetus, as well as causing infertility.
  • Have abnormal blood flow to the uterus, usually pre-eclampsia or cholestasis; This occurs when one or both uterine arteries restrict the flow of oxygen and vital nutrients to the placenta, compromising the growth of the baby.
  • Have low blood levels of the immune system protein, macrophage inhibitory cytokine 1 (MIC 1), which often shows up to 3 weeks before a miscarriage. It also affects foetal viability in early pregnancy, suggesting possible predictive and causative roles.
  • Have unsupportive or negative beliefs around her own fertility, ability to carry a baby, or feel fundamentally unsafe in the world. Marisa Peer is a speaker, therapist and behavioural expert. She has spent nearly three decades treating a client list that includes international superstars, CEOs, Royalty, and Olympic athletes. She has helped thousands of people to overcome profound personal issues. In this video she talks about how the brain works and reveals her fundamental rule—that all our emotional and personal problems come from us believing that we’re not ‘enough’—and explains how to overcome it, the results are tremendous and dramatic. The video is 43 mins long but is well worth listening to the whole thing.
  • Have Rhesus negative blood type: RH – or RH + problems leading to antibodies being formed which can also affect subsequent pregnancies. Anti-D lg must be given with-in 72 hours if the woman is Rhesus negative and has suffered a miscarriage, had an abortion, an Ectopic pregnancy or a stillbirth. Anti-D lg should also be given at birth if the baby is Rhesus positive.
  • Have a family history of reproductive problems. This is usually connected with previous multiple losses and/or later, LLETZ treatment for CIN (basically, abnormal cells in the cervix) 09.09 see https://www.colposcopy.co.uk/treatment.htm
    Scientists have identified that the gene NOS3 appears to trigger repeated unexplained miscarriages in some women. People with a gene mutation in the enzyme MTHFR (methylenetetrahydrofolate reductase), which is involved in amino acid metabolism in the body, can decrease the ability to metabolize folic acid and other B vitamins.
    (There is ongoing speculation around both the latter entries so you may wish to research the latest information yourselves.)
  • Article on the Risks and Demands of Pregnancy After 20
  • May have high or low copper and/or high or low oestrogen levels. See link
  • Diseases or infections

    • Endometriosis; this can double the chances of miscarriage as it creates a hostile environment for the baby to grow in… treatment can be helpful.
    • Hypothyroidism; this has now been connected to late miscarriage but if known about or tested for it can be dealt with. Personal history and shared care with an obstetrician is advisable (Thyroxine).
    • Diabetes; this devastating disease also causes abnormalities in the babies development. It can be treated with the anti-diabetes drug ‘Metformin’ which appears to reduce the likelihood of early miscarriage.
    • ‘Celiac Disease; Quote from the following website which has more information
      https://www.greenmedinfo.com/article/recurrent-miscarriages-may-occur-due-undiagnosed-celiac-disease-and-may. Available literature data shows that celiac disease (CD) is a frequent cause of recurrent miscarriage. However, data is lacking for pregnancy outcomes when the patient is on a gluten-free diet (GFD). A case-control study about the effect of GFD on pregnancy was conducted from 1995 to 2006. A cohort of 13 women (mean age 32 years, range 22-38 years) affected by CD with recurrent miscarriages was observed. In all of them several causes of miscarriage (gynecological, endocrine, hematological, etc.) were excluded. All patients were started on a gluten-free diet and were reassessed throughout a long-term follow-up period to evaluate the outcome of pregnancy.’
    • Kidney & congenital heart (obstetric high risk), Colitis (Auto-immune disease where a severe bout can starve baby or body not cope with both), bacterial vaginosis (can cause late miscarriage), Scleroderma (a soft tissue disease) skin and joint diseases.
    • Thrombophilic/Blood Clotting Disorders which should have already been picked up or found in the first round of ante-natal blood tests; ‘Anti phospholipid Syndrome’ (or Hughes Syndrome), Lupus, PAPS Syndrome which are related to repeat miscarriages. Treatment with Aspirin, Clexane or Heparin works in many cases. Subsequent pregnancies may not have disorders, hence auto-immune response to pregnancy, however midwife and obstetric care are recommended throughout (IUGR/IUD).
    • Any severe infection such as rubella, cytomegalovirus, syphilis, mycoplasma, toxoplasmosis, gonorrhoea, Chlamydia, malaria or Listeria contracted from certain foods (see your LMC for updated info.) The baby is rarely carried to full term when it is then likely to be severely handicapped.
    • May have a uterine infection, bacterial vaginosis (this showed in one study that women with this infection were 9 times more likely to miscarry than uninfected women). Also certain viruses like ‘slapped cheek’ or ‘Fifth disease’ infection. (This last is best diagnosed early and will need Anti-biotics in labour if baby carried to full term.)

    Lifestyle Risks

    • Career – there is anecdotal evidence that women air hostesses, dental nurses, veterinarians and those doing continuous heavy lifting are at greater risk. Tiredness and excessive work hours (e.g. 60 hours per week) and continual nightshifts can lead to an unhealthy lifestyle stopping the body from being pregnancy friendly.
    • Having multiple D&C’s (Dilation & Curettage)
    • Using spas and saunas or exercising excessively causing undesirable high temperatures
    • Using pain relieving medication such as non-steroidal anti-inflammatory drugs like ibuprofen and aspirin may increase risk around the time of conception. These are also blood thinning agents.
    • Unhealthy lifestyles like a poor diet
    • Science Daily (Dec. 4, 2006); Women who have a low body mass index before they become pregnant are 72% more likely to suffer a miscarriage in the first three months of pregnancy. They can reduce the risk significantly by taking supplements and eating fresh fruit and vegetables, according to study findings published online today.
    • Comments from the Recurrent Miscarriage Clinic, Sheffield Hospital, UK. (07.09); ‘Being clinically obese or underweight has a small but significant increased risk of miscarriage in subsequent pregnancies.’


    This is the 2013 ‘Guideline of things to avoid during pregnancy’ especially relating to miscarriage. They have changed over the years, particularly with alcohol and food, some foods now acceptable and others to avoid added. (Checking out with your medical professional for any updates is advisable)

    Things to avoid during pregnancy;

    • Food; Do not eat any of the following foods:
      Soft cheese like camembert, brie, feta, blue, mozzarella, ricotta (except in small quantities and eaten directly and immediately from manufacturer’s wrapping – do not reseal & eat), humus and other dips containing tahini, salads including rice, pasta, coleslaw, roasted vegetable and green salads and also sandwiches (unless prepared at home), aioli, raw eggs (which are also in mayonnaise and hollandaise sauce); desserts such as mousse; cold cooked or smoked chicken, rare meat, pate, liver, hummus, sushi, deli food, custard, cream, raw milk, soft serve ice-cream, yogurt more than 2 days old, ice, water (if travelling overseas), and energy drinks. Chocolate – kept to a minimum to help guard against unnecessary weight gain as well.
      Seafood; any raw seafood, Bluff and Pacific oysters, blue fish tuna & marlin. Smoked or pre-cooked fish including sushi, smoked salmon and marinated mussels or oysters.
      Fish Options: Many kinds of fish, including snapper, may be eaten but no more than 3 or 4 times a week.
      (Quotes from both NZ Herald 28.07.13 & Ministry of Health)
    • Caffeine; The British Medical Journal linked caffeine to an increase of restricted foetal growth and the British Food Standards Agency added the risk of miscarriage. They rank caffeine with alcohol in its potential to harm unborn children. Pregnant women are being advised to cut their coffee intake to 2 mugs per day and are warned against ‘energy’ and ‘smart drinks’ (including soft drinks – American FDA – check information on bottles) saying that these contained other ingredients beside caffeine which are not recommended for pregnant women. Ordinary tea contains caffeine as well. It also warns to be ‘cautious’ about herbal teas. Abstaining is a good option, especially if a miscarriage has already been experienced.
    • Dehydration; in general the more water that is drunk is beneficial anyway (as opposed to other sorts of drinks). Being aware of your intake per day is helpful for avoiding miscarriage as it seems to be accepted by medical research that lack of water is a contributor to it. The amount to drink varies according to what website you access however at least 8 glasses sounds reasonable. When taking the baby’s needs, morning sickness, hot weather and exercise into account, you may wish to drink more. (Excessive thirst can be an indication of diabetes or a more serious condition).
    • Alcohol; Using it can cause Fetal Alcohol Syndrome, IVGR. Most medical recommendations are to drink none at all.
    • Drugs; Possible miscarriage is linked to certain prescription drugs especially those used to treat acne (e.g. Accutane), malaria and cancer as they alter the blood structure. Exposure to DES (diethylstilbestrol) in the womb and foetal exposure to DES has also been associated with a weak cervix. Check out with your doctor any over-the-counter drugs for suspected adverse effects before taking during pregnancy. Using recreational drugs any time may affect your baby and this applies to both partners pre-pregnancy.


    Environmental pollution e.g. exposure to noxious or toxic substances like; anaesthetic gases, dry cleaning fluids, all solvents, petroleum products, arsenic, oxide, lead, mercury (source may be amalgam dental fillings), formaldehyde, Roundup, radiation and chemical by-products in tap water, trihalomethanes can all cause fetal abnormality as well as miscarriage (both partners). Concern around BPA being linked to health risks like breast cancer and miscarriages, amongst other things, may have no proven foundation but it would be circumspect to err on the side of caution. In July 2005, New Zealand TV One News had an item about miscarriage regarding women living near an electro magnetic field (EMF). It stated that 80% of them would miscarry. This conclusion is seen as controversial on many internet sites. The UK National Radiology Protection Board says that miscarriage could affect 5% to 10% of women.

    The Father

    • Research shows that when a body is exposed to environmental pollution it can damage the sperm. The latest research is that up to 50% of miscarriages maybe a result of the state of the father’s sperm. (Research reported in News Post India and Google in June about male partners of women who appeared healthy but were recurrent miscarriers, showed an enzyme at the top of the sperm was defective affecting 80% of sperm. The statistic startled researchers and caused others to query. They blamed environmental pollution. There is also a book called ‘Our Stolen Future’ that is interesting.) Sometimes DNA in sperm that initially tests normal can fragment later. This is caused by free radical damage for ‘Environmental and consumption’ reasons. Internet sources suggest possible treatment using antioxidants and 1gm of Vit. C and 1 gm of Vit. E for 2 months can reduce the incidence of sperm DNA fragmentation markedly and avoid re-current miscarriages.
    • Genetic abnormalities and repeat miscarriages can be chromosomal disorders. The exact cause can possibly be determined with a geneticist referral and the use of blood tests (karyotype) to study the sperm.
    • The effects of different things on men’s sperm production is being researched and a couple are;
      a) a good diet can make a big difference in the amount of sperm produced. When there was a low sperm count, the better the diet the more produced. b) working with a lap-top on a lap rather than on a table, could mean that the genital area is affected by the higher temperature, which can lower the amount of sperm produced. The electrical transference can interfere with the body cell development.

    Age and Miscarriage

    Recent statistics in NZ give the average age of first time mothers as late as 30 years of age (35 in the USA) which adds to a higher baby loss rate. Miscarriages increase after 27 years of age, slowly at first, gaining momentum after 35 up to 25%. Even then, the chances of having a child are still reasonable. Once past 40 years of age however, the statistics progress to 33% and over 45 to when at least 50% (and a recent figure @ 06.16 suggests 93%) of pregnancies miscarry. It is still possible however and happens, although the successful birth of babies to older high profile women in the news. Often this is after IVF treatment, (‘in vitro fertilisation’ the outcome commonly known as a ‘test tube baby’) or a miscarriage, which we don’t necessarily hear about.


    The reason is that the closer to menopause and the eggs expiry date, the poorer the quality and the more chromosomal abnormalities possible. (Chromosomes are tiny structures inside the cells of the body which carry many genes. Genes determine all of a person’s physical attributes, such as sex, hair, eye colour and blood type.) The incidence of ‘Down Syndrome’ (or any other syndrome) is also more likely to occur in women over 30 years of age with the higher risk of multiple miscarriages. If the pregnancy is the result of IVF, then it is possible for the zygote (from the newly combined egg and sperm) to be tested prior to implantation. This is called the FISH test.


    Women with a history of problem pregnancies may request IVF. These pregnancies can progress normally and the care will be no different than that of a woman who has conceived naturally. In an interview with the English Lord Robert Winston, (the authority on reproduction in the UK), said “Couples who were trying for babies over the age of 40, had as much chance of conceiving naturally as through IVF.”

    The physiology for women is: 3 months before women are born they have the maximum number of eggs, approximately 2 million. At puberty, about 300,000 eggs are left. Eggs are constantly being lost, so at around 37, there are only approximately 30,000 eggs remaining.  At menopause a woman would be lucky to have 1,000 left and their quality would be negligible.
    For men: Lord Winston said, “However, men go on producing sperm at a faster rate.” He did not comment on their quality. Recent research suggests a male’s age can also be a miscarriage factor as it is in women. In general, researchers found that miscarriage risk steadily inched upward as men grew older, doubling between the ages of 20 and 50. They also found males are responsible for 40% of miscarriages.


    Fertility also may be difficult for some women. The probability of having a baby decreases 3-5% per year after the age of 30, and like miscarriage, at a faster rate after 40. Research shows problems caused by men’s age run reasonably parallel to women’s. An older father may also mean problems like notably reduced sperm production, movement and quality, which is on the increase in western societies. Like women, there are always exceptions and normal babies are delivered. https://www.fertilitynz.org.nz/

    Pregnant women over 35-years-old are classified at-risk for Down’s syndrome and other chromosomal abnormalities.  At 15 weeks they are offered an amniocentesis test to rule them out. (see Scans)

    Recurrent Miscarriage video on behalf of Fertility NZ