Pregnant women are more severely affected by influenza, hepatitis E, herpes simplex and malaria. The evidence is more limited for coccidioidomycosis, measles, smallpox, and varicella. Pregnancy may also increase susceptibility for toxoplasmosis.

During the 2009 H1N1 pandemic, as well as during interpandemic periods, women in the third trimester of pregnancy were at increased risk for severe

disease, such as disease requiring admission to an intensive care unit or resulting in death, as compared with women in an earlier stage of pregnancy.

For hepatitis E, the case fatality rate among pregnant women has been estimated to be between 15% and 25%, as compared with a range of 0.5 to 4% in the population overall, with the highest susceptibility in the third trimester.

Primary herpes simplex infection, when occurring in pregnant women, has an increased risk of dissemination and hepatitis, an otherwise rare complication in immunocompetent adults, particularly during the third trimester. Also, recurrences of herpes genitalis increase in

frequency during pregnancy.

The risk of severe malaria by "Plasmodium falciparum" is three times as high in pregnant women, with a median maternal mortality of 40% reported in studies in the Asia–Pacific region. In women where the pregnancy is not the first, malaria infection is more often asymptomatic, even at high parasite loads, compared to women having their first pregnancy. There is a decreasing susceptibility to malaria with increasing parity, probably due to immunity to pregnancy-specific antigens. Young maternal age and increases the risk. Studies differ whether the risk is different in different . Limited data suggest that malaria caused by "Plasmodium vivax" is also more severe during pregnancy.

Severe and disseminated coccidioidomycosis has been reported the occur in increased frequency in pregnant women in several reports and case series, but subsequent large surveys, with the overall risk being rather low.

Varicella occurs at an increased rate during pregnancy, but mortality is not higher than that among men and non-pregnant women.

Listeriosis mostly occurs during the third trimester, with Hispanic women appearing to be at particular risk. Listeriosis is a vertically transmitted infection that may cause miscarriage, stillbirth, preterm birth, or serious neonatal disease.

Some infections are vertically transmissible, meaning that they can affect the child as well.

Early onset sepsis can occur in the first week of life. It usually is apparent on the first day after birth. This type of infection is usually acquired before the birth of the infant. Premature rupture of membranes and other obstetrical complications can add to the risk of early-onset sepsis. If the amniotic membrane has been ruptured greater than 18 hours before delivery the infant may be at more risk for this complication. Prematurity, low birth weight, chorioamnionitis, maternal urinary tract infection and/or maternal fever are complications that increase the risk for early-onset sepsis. Early onset sepsis is indicated by serious respiratory symptoms. The infant usually suffers from pneumonia, hypothermia, or shock. The mortality rate is 30 to 50%.

Human immunodeficiency virus type I (HIV) infection can occur during labor and delivery, in utero through mother-to-child transmission or postnatally by way of breastfeeding. Transmission can occur during pregnancy, delivery or breastfeeding. Most transmission occurs during delivery. In women with low detectable levels of the virus, the incidence of transmission is lower. Transmission risk can be reduced by:

- providing antiretroviral therapy during pregnancy and immediately after birth

- delivery by caesarean section

- not breastfeeding

- antiretroviral prophylaxis in infants born to mothers with HIV.

A low number of women whose HIV status are unknown until after the birth, do not benefit from interventions that could help lower the risk of mother-to-child HIV transmission.

Pregnant women with HIV may still receive the trivalent inactivated influenza vaccine and the tetanus, diphtheria, and pertussis (Tdap) vaccination during pregnancy.

Many patients who are HIV positive also have other health conditions known as comorbidities. Hepatitis B, hepatitis C, tuberculosis and injection drug use are some of the most common comorbidities associated with HIV. Women who screen positive for HIV should also be tested for these conditions so that they may be adequately treated or controlled during the pregnancy. The comorbidities may have serious adverse effects on the mother and child during pregnancy, so it is extremely important to identify them early during the pregnancy.

Women may transmit HIV to their child via breastmilk. For this reason, breastfeeding is discouraged amongst HIV-positive women. In a study conducted in South Africa, 14.1% of children born to HIV-infected mothers were infected within 6 weeks of breastfeeding and 19.5% were infected by 6 months of age. A study in Malawi found that the risk of HIV transmission through breastfeeding was 7% in children who breastfed for one year and 10% in children who breastfed for two years. The risk of HIV infection appears to be highest in the early months of breastfeeding and HIV-infected mothers should avoid breastfeeding entirely if possible.

In developed countries where clean water and infant formula are both accessible and available, HIV-positive women should not breastfeed. They should use formula to reduce the risk of transmitting HIV to the child. Even if the mother is on ART, she should avoid breastfeeding as HIV can still be transmitted through the breastmilk. Some women elect to use donor milk (breast milk donated from non-HIV infected mothers) instead of formula so that their child may receive the health benefits of breast milk, the most notable being increased immunity.

In underdeveloped countries where clean water and formula are not available, breastfeeding is encouraged to provide the child with adequate food and nutrients. The benefit of nourishment outweighs the risk of HIV transmission, malnutrition, and other infections and so breastfeeding is acceptable.

Most healthy people working with infants and children face no special risk from CMV infection. However, for women of child-bearing age who previously have not been infected with CMV, there is a potential risk to the developing unborn child (the risk is described above in the Pregnancy section). Contact with children who are in day care, where CMV infection is commonly transmitted among young children (particularly toddlers), may be a source of exposure to CMV. Since CMV is transmitted through contact with infected body fluids, including urine and saliva, child care providers (meaning day care workers, special education teachers, as well as mothers) should be educated about the risks of CMV infection and the precautions they can take. Day care workers appear to be at a greater risk than hospital and other health care providers, and this may be due in part to the increased emphasis on personal hygiene in the health care setting.

Recommendations for individuals providing care for infants and children:

- Employees should be educated concerning CMV, its transmission, and hygienic practices, such as handwashing, which minimize the risk of infection.

- Susceptible nonpregnant women working with infants and children should not routinely be transferred to other work situations.

- Pregnant women working with infants and children should be informed of the risk of acquiring CMV infection and the possible effects on the unborn child.

- Routine laboratory testing for CMV antibody in female workers is not specifically recommended due to its high occurrence, but can be performed to determine their immune status.

Recommendations for pregnant women with regard to CMV infection:

- Throughout the pregnancy, practice good personal hygiene, especially handwashing with soap and water, after contact with diapers or oral secretions (particularly with a child who is in day care). Sharing of food, eating and drinking utensils, and contact with toddlers' saliva should be avoided.

- Women who develop a mononucleosis-like illness during pregnancy should be evaluated for CMV infection and counseled about the possible risks to the unborn child.

- Laboratory testing for antibody to CMV can be performed to determine if a woman has already had CMV infection.

- Recovery of CMV from the cervix or urine of women at or before the time of delivery does not warrant a cesarean section.

- The demonstrated benefits of breast-feeding outweigh the minimal risk of acquiring CMV from the breast-feeding mother.

- There is no need to either screen for CMV or exclude CMV-excreting children from schools or institutions because the virus is frequently found in many healthy children and adults.

Treatment with hyperimmune globulin in mothers with primary CMV infection has been shown to be effective in preventing congenital disease in several studies. One study did not show significant decrease in the risk of congenital cytomegalovirus infection.

There are several potential risk factors or causes to this increased risk:

- An increased immune tolerance in pregnancy to prevent an immune reaction against the fetus

- Maternal physiological changes including a decrease in respiratory volumes and urinary stasis due to an enlarging uterus.

- The presence of a placenta for pathogens to use as a habitat, such as by "L. monocytogenes" and "P. falciparum".

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.

The embryo and fetus have little or no immune function. They depend on the immune function of their mother. Several pathogens can cross the placenta and cause (perinatal) infection. Often, microorganisms that produce minor illness in the mother are very dangerous for the developing embryo or fetus. This can result in spontaneous abortion or major developmental disorders. For many infections, the baby is more at risk at particular stages of pregnancy. Problems related to perinatal infection are not always directly noticeable.

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.

Babies can also become infected by their mothers during birth. Some infectious agents may be transmitted to the embryo or fetus in the uterus, while passing through the birth canal, or even shortly after birth. The distinction is important because when transmission is primarily during or after birth, medical intervention can help prevent infections in the infant.

During birth, babies are exposed to maternal blood, body fluids, and to the maternal genital tract without the placental barrier intervening. Because of this, blood-borne microorganisms (hepatitis B, HIV), organisms associated with sexually transmitted disease (e.g., "Neisseria gonorrhoeae" and "Chlamydia trachomatis"), and normal fauna of the genitourinary tract (e.g., "Candida albicans") are among those commonly seen in infection of newborns.

In the western world, GBS (in the absence of effective prevention measures) is the main cause of bacterial infections in newborns, such as septicemia, pneumonia, and meningitis, which can lead to death or long-term after effects.

GBS infections in newborns are separated into two clinical types, early-onset disease (GBS-EOD) and late-onset disease (GBS-LOD). GBS-EOD manifests from 0 to 7 living days in the newborn, most of the cases of EOD being apparent within 24 h from birth. GBS-LOD starts between 7 and 90 days after birth.

The most common clinical syndromes of GBS-EOD are septicemia without apparent location, pneumonia, and less frequently meningitis. Bacteremia without a focus occurs in 80-85%, pneumonia in 10-15%, and meningitis in 5-10% of cases. The initial clinical findings are respiratory signs in more than 80% of cases. Neonates with meningitis often have an initial clinical presentation identical to presentation in those without meningeal affectation. An exam of the cerebrospinal fluid is often necessary to rule out meningitis.

Colonization with GBS during labour is the primary risk factor for the development of GBS-EOD. GBS-EOD is acquired vertically (vertical transmission), through exposure of the fetus or the baby to GBS from the vagina of a colonized woman, either "in utero" (because of ascending infection) or during birth, after rupture of membranes. Infants can also be infected during passage through the birth canal, nevertheless, newborns who acquire GBS through this route can only become colonized, and these colonized infants usually do not develop GBS-EOD.

Roughly 50% of newborns of GBS colonized mothers are also GBS colonized and (without prevention measures) 1-2% of these newborns will develop GBS-EOD.

In the past, the incidence of GBS-EOD ranged from 0.7 to 3.7 per thousand live births in the US, and from 0.2 to 3.25 per thousand in Europe.

In 2008, after widespread use of antenatal screening and intrapartum antibiotic prophylaxis, the Centers for Disease Control and Prevention of United States reported an incidence of 0.28 cases of GBS-EOD per thousand live births in the US.

Though maternal GBS colonization is the key determinant for GBS-EOD, other factors also increase the risk. These factors are:

- Onset of labour before 37 weeks of gestation (premature birth)

- Prolonged rupture of membranes (longer duration of membrane rupture) (≥18 h before delivery)

- Intrapartum (during childbirth) fever (>38 °C, >100.4 °F)

- Amniotic infections (chorioamnionitis)

- Young maternal age

Nevertheless, most babies who develop GBS-EOD are born to colonized mothers without any of these risk factors. Heavy GBS vaginal colonization is also associated with a higher risk for GBS-EOD. Women who had one of these risk factors but who are not GBS colonized at labour are at low risk for GBS-EOD compared to women who were colonized prenatally, but had none of the aforementioned risk factors.

Presence of low levels of anticapsular antibodies against GBS in the mother are also of great importance for the development of GBS-EOD.

Because of that, a previous sibling with GBS-EOD is also an important risk factor for the development of the infection in subsequent deliveries, probably reflecting the lack of protective antibodies in the mother.

Overall, the case fatality rates from GBS-EOD have declined, from 50% observed in studies from the 1970s to between 2 and 10% in recent years, mainly as a consequence of improvements in therapy and management. Fatal neonatal infections by GBS are more frequent among premature infants.

GBS-LOD affects infants from 7 days to 3 months of age and has a lower case fatality rate (1%-6%) than GBS-EOD. Clinical syndromes of GBS-EOD are bacteremia without a focus (65%), meningitis (25%), cellulitis, osteoarthritis, and pneumonia.

Prematurity has been reported to be the main risk factor. Each week of decreasing gestation increases the risk by a factor of 1.34 for developing GBS-LOD.

GBS-LOD is not acquired through vertical transmission during delivery; it can be acquired later from the mother from breast milk or from environmental and community sources.

GBS-LOD commonly shows nonspecific signs, and diagnosis should be made obtaining blood cultures in febrile newborns. Hearing loss and mental impairment can be a long-term consequence of GBS meningitis.

Factors increasing the risk (to either the woman, the fetus/es, or both) of pregnancy complications beyond the normal level of risk may be present in a woman's medical profile either before she becomes pregnant or during the pregnancy. These pre-existing factors may relate to physical and/or mental health, and/or to social issues, or a combination.

Some common risk factors include:

- Age of either parent

- Adolescent parents

- Older parents

- Exposure to environmental toxins in pregnancy

- Exposure to recreational drugs in pregnancy:

- Ethanol during pregnancy can cause fetal alcohol syndrome and fetal alcohol spectrum disorder.

- Tobacco smoking and pregnancy, when combined, causes twice the risk of premature rupture of membranes, placental abruption and placenta previa. Also, it causes 30% higher odds of the baby being born prematurely.

- Prenatal cocaine exposure is associated with, for example, premature birth, birth defects and attention deficit disorder.

- Prenatal methamphetamine exposure can cause premature birth and congenital abnormalities. Other investigations have revealed short-term neonatal outcomes to include small deficits in infant neurobehavioral function and growth restriction when compared to control infants. Also, prenatal methamphetamine use is believed to have long-term effects in terms of brain development, which may last for many years.

- Cannabis in pregnancy is possibly associated with adverse effects on the child later in life.

- Exposure to Pharmaceutical drugs in pregnancy. Anti-depressants, for example, may increase risks of such outcomes as preterm delivery.

- Ionizing radiation

- Risks arising from previous pregnancies:

- Complications experienced during a previous pregnancy are more likely to recur.

- Many previous pregnancies. Women who have had five previous pregnancies face increased risks of very rapid labor and excessive bleeding after delivery.

- Multiple previous fetuses. Women who have had more than one fetus in a previous pregnancy face increased risk of mislocated placenta.

- Multiple pregnancy, that is, having more than one fetus in a single pregnancy.

- Social and socioeconomic factors. Generally speaking, unmarried women and those in lower socioeconomic groups experience an increased level of risk in pregnancy, due at least in part to lack of access to appropriate prenatal care.

- Unintended pregnancy. Unintended pregnancies preclude preconception care and delays prenatal care. They preclude other preventive care, may disrupt life plans and on average have worse health and psychological outcomes for the mother and, if birth occurs, the child.

- Height. Pregnancy in women whose height is less than 1.5 meters (5 feet) correlates with higher incidences of preterm birth and underweight babies. Also, these women are more likely to have a small pelvis, which can result in such complications during childbirth as shoulder dystocia.

- Weight

- Low weight: Women whose pre-pregnancy weight is less than 45.5 kilograms (100 pounds) are more likely to have underweight babies.

- Obese women are more likely to have very large babies, potentially increasing difficulties in childbirth. Obesity also increases the chances of developing gestational diabetes, high blood pressure, preeclampsia, experiencing postterm pregnancy and/or requiring a cesarean delivery.

- Intercurrent disease in pregnancy, that is, a disease and condition not necessarily directly caused by the pregnancy, such as diabetes mellitus in pregnancy, SLE in pregnancy or thyroid disease in pregnancy.

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.

Though GBS colonization is asymptomatic and, in general, does not cause problems, it can sometimes cause serious illness for the mother and the baby during gestation and after delivery. GBS infections in the mother can cause chorioamnionitis (intra-amniotic infection or severe infection of the placental tissues) infrequently, and postpartum infections (after birth). GBS urinary tract infections may induce labour and cause premature delivery (preterm birth) and miscarriage.

Some disorders and conditions can mean that pregnancy is considered high-risk (about 6-8% of pregnancies in the USA) and in extreme cases may be contraindicated. High-risk pregnancies are the main focus of doctors specialising in maternal-fetal medicine.

Serious pre-existing disorders which can reduce a woman's physical ability to survive pregnancy include a range of congenital defects (that is, conditions with which the woman herself was born, for example, those of the heart or , some of which are listed above) and diseases acquired at any time during the woman's life.

Several intercurrent diseases in pregnancy can potentially increase the risk of miscarriage, including diabetes, polycystic ovary syndrome (PCOS), hypothyroidism, certain infectious diseases, and autoimmune diseases. PCOS may increases the risk of miscarriage. Two studies suggested treatment with the drug metformin significantly lowers the rate of miscarriage in women with PCOS, but the quality of these studies has been questioned. The use metformin treatment in pregnancy has not been shown to be safe. In 2007 the Royal College of Obstetricians and Gynaecologists also recommended against use of the drug to prevent miscarriage. Thrombophilias or defects in coagulation and bleeding were once thought to be a risk in miscarriage but have been subsequently questioned.

Severe cases of hypothyroidism increase the risk of miscarriage. The effect of milder cases of hypothyroidism on miscarriage rates has not been established. A condition called luteal phase defect (LPD) is a failure of the uterine lining to be fully prepared for pregnancy. This can keep a fertilized egg from implanting or result in miscarriage.

"Mycoplasma genitalium" infection is associated with increased risk of preterm birth and miscarriage.

Infections can increase the risk of a miscarriage: rubella (German measles), cytomegalovirus, bacterial vaginosis, HIV, chlamydia, gonorrhoea, syphilis, and malaria.

PROM occurring before 37 weeks (PPROM) is one of the leading causes of preterm birth. 30-35% of all preterm births are caused by PPROM. This puts the fetus at risk for the many complications associated with prematurity such as respiratory distress, brain bleeds, infection, necrotizing enterocolitis (death of the fetal bowels), brain injury, muscle dysfunction, and death. Prematurity from any cause leads to 75% of perinatal mortality and about 50% of all long-term morbidity. PROM is responsible for 20% of all fetal deaths between 24 and 34 weeks gestation.

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.

Chorioamnionitis is a risk factor for periventricular leukomalacia and cerebral palsy.

Before 24 weeks the fetus is still developing its organs, and the amniotic fluid is important for protecting the fetus against infection, physical impact, and for preventing the umbilical cord from becoming compressed. It also allows for fetal movement and breathing that is necessary for the development of the lungs, chest, and bones. Low levels of amniotic fluid due to mid-trimester or previable PPROM (before 24 weeks) can result in fetal deformity (ex: Potter-like facies), limb contractures, pulmonary hypoplasia (underdeveloped lungs), infection (especially if the mother is colonized by group B streptococcus or bacterial vaginosis), prolapsed umbilical cord or compression, and placental abruption.

Doxycycline is the drug of choice, but azithromycin is also used as a five-day course rather than a single dose that would be used to treat "Chlamydia" infection; streptomycin is an alternative, but is less popular because it must be injected. Penicillins are ineffective — "U. urealyticum" does not have a cell wall, which is the drug's main target.

Infection in the newborn is accompanied by a strong immune response and is correlated with the need for prolonged mechanical ventilation.

Infection with "U. urealyticum" in pregnancy and birth can be complicated by chorioamnionitis, stillbirth, premature birth, and, in the perinatal period, pneumonia, bronchopulmonary dysplasia and meningitis. "U. urealyticum" has been found to be present in amniotic fluid in women who have had a premature birth with intact fetal membranes.

"U. urealyticum" has been noted as one of the infectious causes of sterile pyuria. It increases the morbidity as a cause of neonatal infections. It is associated with premature birth, preterm rupture of membranes, preterm labor, cesarean section, placental inflammation, congenital pneumonia, bacteremia, meningitis, fetal lung injury and death of infant. "Ureaplasma urealyticum" is associated with miscarriage.

The disease results from the aggregation of erythrocytes infected by "Plasmodium falciparum" which have been shown to adhere to chondroitin sulfate A (CSA) on placental proteoglycans causing them to accumulate in the intervillous spaces of the placenta, blocking the crucial flow of nutrients from mother to embryo.