Tobacco (cigarette) smokers have an increased risk of miscarriage. There is an increased risk regardless of which parent smokes, though the risk is higher when the gestational mother smokes.

Not only is obesity associated with miscarriage, it can result in sub-fertility and other adverse pregnancy outcomes. Recurrent miscarriage is also related to obesity. Women with bulimia nervosa and anorexia nervosa may have a greater risk for miscarriage. Nutrient deficiencies have not been found to impact miscarriage rates but hyperemesis gravidarum sometimes precedes a miscarriage.

Caffeine consumption also has been correlated to miscarriage rates, at least at higher levels of intake. However, such higher rates have been found to be statistically significant only in certain circumstances.

Vitamin supplementation has generally not shown to be effective in preventing miscarriage. Chinese traditional medicine has not been found to prevent miscarriage.

The term "toxic abortion" was first used to identify this phenomenon in humans in the earliest studies of the effects of pollutants on pregnancy in 1928, "An Experimental Investigation Concerning Toxic Abortion Produced by Chemical Agents" by Morris M. Datnow M.D.

Toxic abortion chemicals studied at that time were:

Petrochemicals,

Heavy metals,

Organic solvents,

Tetrachloroethylene,

Glycol ethers,

2-Bromopropane,

Ethylene oxide,

Anesthetic gases, and

Antineoplastic drugs.

In 1932, the "Journal of State Medicine" reported on a natural variation, with the occurrence of "a considerable number of cases of toxic abortion" being caused by untreated dental caries.

Study of pollution-caused abortion in humans ceased for a considerable time, interest renewing in the 2000s. A 2009 study found that fossil fuels play a role, as "pregnant African-American women who live within a half mile of freeways and busy roads were three times more likely to have miscarriages than women who don't regularly breathe exhaust fumes." A 2011 study found a correlation between exposure to workplace toxins and spontaneous abortion, and called for further study. "Newsweek" magazine reported in May 2014 that a spike in stillborn babies in the town of Vernal, in Utah, had correlated with an increase in pollution from new gas and oil drilling. "Newsweek" reported that "Vernal’s rate of neonatal mortality appears to have climbed from about average in 2010 (relative to national figures) to six times the normal rate three years later." "Newsweek" quoted one expert's observation that "We know that pregnant women who breathe more air pollution have much higher rates of virtually every adverse pregnancy outcome that exists." A study published in the "Journal of Environmental Health" in October 2014 found tetrachloroethylene or PCE, to be "linked to increased risk for stillbirths and other pregnancy complications."

The PCE study found that "pregnancies with high exposure to PCE were 2.4 times more likely to end with stillborn babies and 1.4 times more likely to experience placental abruption — when the placenta peels away from uterine wall before delivery, causing the mother to bleed and the baby to lose oxygen — compared with pregnancies never exposed to PCE." Higher exposure lead to a 35 percent higher risk of abruption. PCE has also been tied to an increased risk for cancer. Children exposed to PCE as fetuses and toddlers are more likely to use drugs later in life. The toxin has been linked to mental illness, an increased risk of breast cancer and some birth defects. It has been tied to anxiety, depression, and impairments in cognition, memory and attention. PCE contamination has been found in the Massachusetts water supply and "on military bases across the country," and "water systems in California and Pennsylvania and have also been found to be contaminated with PCE."

In 2015, "Newsweek" reported that chemicals found in fast food wrappers multiply miscarriage risk by sixteen times.

Some instances have been reported of women intentionally seeking to induce toxic abortion, where circumstances make medical abortion difficult to obtain, by exposing themselves to environmental toxins.

Toxic abortion is observed in both humans and in animals such as cows, hares, and horses. The source notes that animal ingestion of "low quality forage having some toxicity" harms livestock health, especially with cattle and horses, leading to numerous cases of "toxic abortion, gastro-enteritis and abortion with dystrophic and haemorrhagic lesions of the foetus." Cadmium has been identified as a chemical pollutant identified with toxic abortion in animals.

Hypercoagulability in pregnancy is the propensity of pregnant women to develop thrombosis (blood clots). Pregnancy itself is a factor of hypercoagulability (pregnancy-induced hypercoagulability), as a physiologically adaptive mechanism to prevent "post partum" bleeding. However, when combined with an additional underlying hypercoagulable states, the risk of thrombosis or embolism may become substantial.

Being pregnant decreases the risk of relapse in multiple sclerosis; however, during the first months after delivery the risk increases. Overall, pregnancy does not seem to influence long-term disability. Multiple sclerosis does not increase the risk of congenital abnormality or miscarriage.

DES (diethylstilbestrol) is a drug that mimics estrogen, a female hormone. From 1938 until 1971 doctors prescribed this drug to help some pregnant women who had had miscarriages or premature deliveries on the theory that miscarriages and premature births occurred because some pregnant women did not produce enough estrogen naturally to sustain the pregnancy for full term . An estimated 5-10 million pregnant women and the children born during this period were exposed to DES. Currently, DES is known to increase the risk of breast cancer, and cause a variety of birth-related adverse outcomes exposed female offsprings such as spontaneous abortion, second-trimester pregnancy loss, preterm delivery, stillbirth, neonatal death, sub/infertility and cancer of reproductive tissues . DES is an important developmental toxicant which links the fetal basis of adult disease.

Although it is very uncommon, women undergoing surgical abortion after 18 weeks gestation sometimes give birth to a fetus that may survive briefly. Longer term survival is possible after 22 weeks.

If medical staff observe signs of life, they may be required to provide care: emergency medical care if the child has a good chance of survival and palliative care if not. Induced fetal demise before termination of pregnancy after 20–21 weeks gestation is recommended to avoid this.

Death following live birth caused by abortion is given the ; data are identified as either fetus or newborn. Between 1999 and 2013, in the U.S., the CDC recorded 531 such deaths for newborns, approximately 4 per 100,000 abortions.

Methylmercury and inorganic mercury is excreted in human breast milk and infants are particularly susceptible to toxicity due to this compound. The fetus and infant are especially vulnerable to mercury exposures with special interest in the development of the CNS since it can easily cross across the placental barrier, accumulate within the placenta and fetus as the fetus cannot eliminate mercury and have a negative effect on the fetus even if the mother does not show symptoms. Mercury causes damage to the nervous system resulting from prenatal or early postnatal exposure and is very likely to be permanent.

The rate of cancer during pregnancy is 0.02–1%, and in many cases, cancer of the mother leads to consideration of abortion to protect the life of the mother, or in response to the potential damage that may occur to the fetus during treatment. This is particularly true for cervical cancer, the most common type of which occurs in 1 of every 2,000–13,000 pregnancies, for which initiation of treatment "cannot co-exist with preservation of fetal life (unless neoadjuvant chemotherapy is chosen)". Very early stage cervical cancers (I and IIa) may be treated by radical hysterectomy and pelvic lymph node dissection, radiation therapy, or both, while later stages are treated by radiotherapy. Chemotherapy may be used simultaneously. Treatment of breast cancer during pregnancy also involves fetal considerations, because lumpectomy is discouraged in favor of modified radical mastectomy unless late-term pregnancy allows follow-up radiation therapy to be administered after the birth.

Exposure to a single chemotherapy drug is estimated to cause a 7.5–17% risk of teratogenic effects on the fetus, with higher risks for multiple drug treatments. Treatment with more than 40 Gy of radiation usually causes spontaneous abortion. Exposure to much lower doses during the first trimester, especially 8 to 15 weeks of development, can cause intellectual disability or microcephaly, and exposure at this or subsequent stages can cause reduced intrauterine growth and birth weight. Exposures above 0.005–0.025 Gy cause a dose-dependent reduction in IQ. It is possible to greatly reduce exposure to radiation with abdominal shielding, depending on how far the area to be irradiated is from the fetus.

The process of birth itself may also put the mother at risk. "Vaginal delivery may result in dissemination of neoplastic cells into lymphovascular channels, haemorrhage, cervical laceration and implantation of malignant cells in the episiotomy site, while abdominal delivery may delay the initiation of non-surgical treatment."

This depends on the age of the animal affected and the efficiency of its immune system.

Colostral protection lasts up to 5 months of age, after which it decreases to an all-time low to increase yet again at about 12 months of age.

- Prenatal infection: virus travels from infected mother to fetus via the placenta. In this case, the time of gestation determines the result of the infection.

- If the fetus is infected in the first 30 days of fetal life, death and absorption of all, or some of the fetuses may occur. In this case, some immunotolerant healthy piglets may be born.

- If the infection happens at 40 days, death and mummification may occur. Also in this case, some or all the fetuses are involved, i.e. some of the fetuses can be born healthy and immunotolerant, or else carriers of the disease.

- If the viruses crosses the placenta in the last trimester, neonatal death may occur, or the birth of healthy piglets with a protective pre-colostral immunity.

- Postnatal infection (pigs up to 1 year of age): Infection occurs oro-nasally, followed by a viremic period associated with transitory leucopenia.

- Infection in adults (over 1 year of age): These subject would have an active, protective immune system which protects them from future exposures (e.g. mating with an infected male).

Therefore, it is important to note that the virus is particularly dangerous for the sow in her first gestation, which would be at 7–8 months of age, as she would have a particularly low antibody count at this age and could easily contract the virus via copulation.

Hormonal and other changes in pregnancy affect physical performance. In the first three months it is known that a woman’s body produces a natural surplus of red blood cells, which are well supplied with oxygen-carrying hemoglobin, in order to support the growing fetus. A study of athletes before and after pregnancy by Professor James Pivarnik at the Human Energy Research laboratory in Michigan State University has found there is a 60 per cent increase in blood volume and that this could improve the body’s ability to carry oxygen to muscles by up to 30 percent. This would have obvious positive effects on aerobic capacity. Other potential advantages are obtained from the surge in hormones that pregnancy induces, predominantly progesterone and estrogen, but also testosterone, which could increase muscle strength. Increases in hormones like relaxin, which loosens the hip joints to prepare for childbirth, may have a performance-enhancing effect on joint mobility.

Several world records have been set by female athletes shortly after giving birth to their first child. This is accepted as a natural and unintended event.

SLE causes an increased rate of fetal death "in utero" and spontaneous abortion (miscarriage). The overall live-birth rate in SLE patient has been estimated to be 72%. Pregnancy outcome appears to be worse in SLE patients whose disease flares up during pregnancy.

Miscarriages in the first trimester appear either to have no known cause or to be associated with signs of active SLE. Later losses appear to occur primarily due to the antiphospholipid syndrome, in spite of treatment with heparin and aspirin. All women with lupus, even those without previous history of miscarriage, are recommended to be screened for antiphospholipid antibodies, both the lupus anticoagulant (the RVVT and sensitive PTT are the best screening battery) and anticardiolipin antibodies.

Mare reproductive loss syndrome (MRLS) is a syndrome consisting of equine abortions and three related nonreproductive syndromes which occur in horses of all breeds, sexes, and ages. MRLS was first observed in the U.S. state of Kentucky in a three-week period around May 5, 2001, when about 20% to 30% of Kentucky's pregnant mares suffered abortions. A primary infectious cause was rapidly ruled out, and the search began for a candidate toxin. No abortifacient toxins were identified.

In the spring of 2001, Kentucky had experienced an extraordinarily heavy infestation of eastern tent caterpillars (ETCs). An epidemiological study showed ETCs to be associated with MRLS. When ETCs returned to Kentucky in the spring of 2002, equine exposure to caterpillars was immediately shown to produce abortions. Research then focused on how the ETCs produced the abortions. Reviewing the speed with which ETCs produced late-term abortions in 2002 experiments, the nonspecific bacterial infections in the placenta/fetus were assigned a primary driving role. The question then became how exposure to the caterpillars produced these non-specific bacterial infections of the affected placenta/fetus and also the uveitis and pericarditis cases.

Reviewing the barbed nature of ETC hairs (setae), intestinal blood vessel penetration by barbed setal fragments was shown to introduce barbed setal fragments and associated bacterial contaminants into intestinal collecting blood vessels (septic penetrating setae). Distribution of these materials following cardiac output would deliver these materials to all tissues in the body (septic penetrating setal emboli). About 15% of cardiac output goes to the late-term fetus, at which point the septic barbed setal fragments are positioned to penetrate placental tissues which lack an immune response. Bacterial proliferation, therefore, proceeds unchecked and the late-term fetus is rapidly aborted.

Similar events occur with the early-term fetus, but as a much smaller target receiving an equivalently smaller fraction of cardiac output, the early-term fetus is less likely to be "hit" by a randomly distributing setal fragment. Since this MRLS pathogenesis model was first proposed in 2002, other caterpillar-related abortion syndromes have been recognized, most notably equine amnionitis and fetal loss in Australia, and more recently, a long-recognized relationship between pregnant camels eating caterpillars and abortions among the camel pastoralists in the western Sahara.

Because the black cherry tree is the preferred host tree for the eastern tent caterpillar, one approach to prevention is to simply remove the trees from the vicinity of horse farms, which was one of the very first recommendations made concerning MRLS. Next, because the brief time for which the full-grown ETCs are on the ground in the vicinity of pregnant mares, simply keeping pregnant mares out of contact with them is also an effective preventative mechanism. In this regard, one Kentucky horse farm took the approach of simply muzzling mares during an ETC exposure period, an approach which was reportedly effective.

No effective treatment for MRLS is apparent. Mares which aborted are treated with broad-spectrum antibiotics to avoid bacterial infections. The foals born from mares infected with MRLS are given supportive care and supplied with medication to reduce inflammatory response and improve blood flow, but none of the treatments appears to be effective, as the majority of the foals do not survive. Unilateral uveitis is treated symptomatically with antibiotics and anti-inflammatory drugs.

Abortion doping refers to the rumoured practice of purposely inducing pregnancy for athletic performance-enhancing benefits, then aborting the pregnancy.

SMEDI (an acronym of stillbirth, mummification, embryonic death, and infertility) is a reproductive disease of swine caused by "Porcine parvovirus" ("PPV") and "Porcine enterovirus". The term SMEDI usually indicates "Porcine enterovirus", but it also can indicate "Porcine parvovirus", which is a more important cause of the syndrome. SMEDI also causes abortion, neonatal death, and decreased male fertility.

From an economic standpoint SMEDI is an important disease because of the loss of productivity from fetal death in affected herds. Initial infection of a herd causes the greatest effect, but losses slow over time. The disease is spread most commonly by ingestion of food and water contaminated with infected feces and occasionally through sexual contact and contact with aborted tissue. A vaccine is available (ATCvet code: ).

Neonatal lupus is the occurrence of SLE symptoms in an infant born from a mother with SLE, most commonly presenting with a rash resembling discoid lupus erythematosus, and sometimes with systemic abnormalities such as heart block or hepatosplenomegaly. Neonatal lupus is usually benign and self-limited. Still, identification of mothers at highest risk for complications allows for prompt treatment before or after birth. In addition, SLE can flare up during pregnancy, and proper treatment can maintain the health of the mother for longer.

The risk of a repeat GTD is approximately 1 in 100, compared with approximately 1 in 1000 risk in the general population. Especially women whose hCG levels remain significantly elevated are at risk of developing a repeat GTD.

Globally, an estimated 125 million or more pregnant women per year risk contracting PAM. Pregnancy-related malaria causes around 100,000 infant deaths each year, due in large part to low birth weight.

Most women with GTD can become pregnant again and can have children again. The risk of a further molar pregnancy is low. More than 98% of women who become pregnant following a molar pregnancy will not have a further hydatidiform mole or be at increased risk of complications.

In the past, it was seen as important not to get pregnant straight away after a GTD. Specialists recommended a waiting period of 6 months after the hCG levels become normal. Recently, this standpoint has been questioned. New medical data suggest that a significantly shorter waiting period after the hCG levels become normal is reasonable for approximately 97% of the patients with hydatidiform mole.

Women with a lifelong epileptic history are also liable to psychoses during labour in the puerperium. Women occasionally develop epilepsy for the first time in relation to their first pregnancy, and psychotic episodes have been described.

Hypothyroidism is diagnosed by noting a high TSH associated with a subnormal T4 concentration. Subclinical hypothyroidism (SCH) is present when the TSH is high but the T4 level is in the normal range but usually low normal. SCH is the commonest form of hypothyroidism in pregnancy and is usually due to progressive thyroid destruction due to autoimmune thyroid disease.

Several studies, mostly retrospective, have shown an association between overt hypothyroidism and adverse fetal and obstetric outcomes (e.g. Glinoer 1991). Maternal complications such as miscarriages, anaemia in pregnancy, pre-eclampsia, abruptio placenta and postpartum haemorrhage can occur in pregnant women with overt hypothyroidism. Also, the offspring of these mothers can have complications such as premature birth, low birth weight and increased neonatal respiratory distress. Similar complications have been reported in mothers with subclinical hypothyroidism. A three-fold risk of placental abruption and a two-fold risk of pre-term delivery were reported in mothers with subclinical hypothyroidism. Another study showed a higher prevalence of subclinical hypothyroidism in women with pre-term delivery (before 32 weeks) compared to matched controls delivering at term. An association with adverse obstetrics outcome has also been demonstrated in pregnant women with thyroid autoimmunity independent of thyroid function. Treatment of hypothyroidism reduces the risks of these adverse obstetric and fetal outcomes; a retrospective study of 150 pregnancies showed that treatment of hypothyroidism led to reduced rates of abortion and premature delivery. Also, a prospective intervention trial study showed that treatment of euthyroid antibody positive pregnant women led to fewer rates of miscarriage than non treated controls.

It has long been known that cretinism (i.e. gross reduction in IQ) occurs in areas of severe iodine deficiency due to the fact that the mother is unable to make T4 for transport to the fetus particularly in the first trimester. This neurointellectual impairment (on a more modest scale) has now been shown in an iodine sufficient area (USA) where a study showed that the IQ scores of 7-9 year old children, born to mothers with undiagnosed and untreated hypothyroidism in pregnancy, were seven points lower than those of children of matched control women with normal thyroid function in pregnancy. Another study showed that persistent hypothyroxinaemia at 12 weeks gestation was associated with an 8-10 point deficit in mental and motor function scores in infant offspring compared to children of mothers with normal thyroid function. Even maternal thyroid peroxidase antibodies were shown to be associated with impaired intellectual development in the offspring of mothers with normal thyroid function. Interestingly, it has been shown that it is only the maternal FT4 levels that are associated with child IQ and brain morphological outcomes, as opposed to maternal TSH levels.

With the great improvement in obstetric care, severe complications of pregnancy and childbirth have become rare. There is, however, a great contrast between Europe, North America, Australia, Japan and some other countries with advanced medical care, and the rest of the world. The wealthiest nations produce only 10 million children each year, from a total of 135 million. They have a maternal mortality rate (MMR) of 6-20/100,000. Some poorer nations with high birth rates have an MMR more than 100 times as high, and, presumably, a corresponding prevalence of severe morbidity. For example, in Sub-Saharan Africa, twenty-one nations, with a combined birth rate of over 14 millions, have an MMR >1,000/100,000. Only a minority of deliveries in sub-Saharan Africa and South Asia are attended by skilled personnel (doctors, nurses or midwives). In Dar es Salaam, the majority of postpartum psychoses are organic. In Africa, India & South East Asia, and Latin America, these organic disorders may still be as important as they have been throughout human history.

A vanishing twin, also known as fetal resorption, is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed. In some instances, the dead twin will be compressed into a flattened, parchment-like state known as "fetus papyraceus".

Vanishing twins occur in up to one out of every eight multifetus pregnancies and may not even be known in most cases. "High resorption rates, which cannot be explained on the basis of the expected abortion rate...suggest intense fetal competition for space, nutrition, or other factors during early gestation, with frequent loss or resorption of the other twin(s)."

In pregnancies achieved by IVF, "it frequently happens that more than one amniotic sac can be seen in early pregnancy, whereas a few weeks later there is only one to be seen and the other has 'vanished'."