Pre-existing disease in pregnancy

A pre-existing disease in pregnancy, is a disease that is not directly caused by the pregnancy in contrast to various complications of pregnancy, but which may become worse or be a potential risk to the pregnancy (such as "causing" pregnancy complications). A major component of this risk can result from necessary use of drugs in pregnancy to manage the disease.
In such circumstances, women who wish to continue with a pregnancy require extra medical care, often from an interdisciplinary team. Such a team might include (besides an obstetrician) a specialist in the disorder and other practitioners (for example, maternal-fetal specialists or obstetric physicians, dieticians, etc.).
Endocrine disorders.
Diabetes mellitus.
Diabetes mellitus and pregnancy deals with the interactions of diabetes mellitus (not restricted to gestational diabetes) and pregnancy. Risks for the child include miscarriage, growth restriction, growth acceleration, fetal obesity (macrosomia), polyhydramnios and birth defects.
Thyroid disease.
Thyroid disease in pregnancy can, if uncorrected, cause adverse effects on fetal and maternal well-being. The deleterious effects of thyroid dysfunction can also extend beyond pregnancy and delivery to affect neurointellectual development in the early life of the child. Demand for thyroid hormones is increased during pregnancy which may cause a previously unnoticed thyroid disorder to worsen. The most effective way of screening for thyroid dysfunction is not known. A review found that more women were diagnosed with thyroid dysfunction when all pregnant women were tested instead of just testing those at ‘high-risk’ of thyroid problems (those with family history, signs or symptoms). Finding more women with thyroid dysfunction meant that the women could have treatment and management through their pregnancies. However the outcomes of the pregnancies were surprisingly similar so more research is needed to look at the effects of screening all women for thyroid problems.
Hypercoagulability.
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 any additional underlying hypercoagulable state, the risk of thrombosis or embolism may become substantial.
Infections.
Vertically transmitted infections.
Many infectious diseases have a risk of vertical transmission to the fetus. Examples include: 
Infections in pregnancy also raise particular concerns about whether or not to use drugs in pregnancy (that is, antibiotics or antivirals) to treat them. For example, pregnant women who contract H1N1 influenza infection are recommended to receive antiviral therapy with either oseltamivir (which is the preferred medication) or zanamivir. Both amantadine and rimantadine have been found to be teratogenic and embryotoxic when given at high doses in animal studies.
Candidal vulvovaginitis.
In pregnancy, changes in the levels of female sex hormones, such as estrogen, make a woman more likely to develop candidal vulvovaginitis. During pregnancy, the "Candida" fungus is more prevalent (common), and recurrent infection is also more likely. There is no clear evidence that treatment of asymptomatic candidal vulvovaginitis in pregnancy reduces the risk of preterm birth. Candidal vulvovaginitis in pregnancy should be treated with intravaginal clotrimazole or nystatin for at least 7 days.
Bacterial vaginosis.
Bacterial vaginosis is an imbalance of naturally occurring bacterial flora in the vagina. Bacterial vaginosis occurring during pregnancy may increase the risk of pregnancy complications, most notably premature birth or miscarriage. However, this risk is small overall and appears more significant in women who have had such complications in an earlier pregnancy.
Valvular heart disease.
In case of valvular heart disease in pregnancy, the maternal physiological changes in pregnancy confer additional load on the heart and may lead to complications.
In individuals who require an artificial heart valve, consideration must be made for deterioration of the valve over time (for bioprosthetic valves) versus the risks of blood clotting in pregnancy with mechanical valves with the resultant need of drugs in pregnancy in the form of anticoagulants.
Other autoimmune disorders.
Celiac disease.
Untreated celiac disease can cause spontaneous abortion (miscarriage), intrauterine growth restriction, small for gestational age, low birthweight and preterm birth. Often reproductive disorders are the only manifestation of undiagnosed celiac disease and most cases are not recognized. Complications or failures of pregnancy cannot be explained simply by malabsorption, but by the autoimmune response elicited by the exposure to gluten, which causes damage to the placenta. The gluten-free diet avoids or reduces the risk of developing reproductive disorders in pregnant women with celiac disease. Also, pregnancy can be a trigger for the development of celiac disease in genetically susceptible women who are consuming gluten.
Systemic lupus erythematosus.
Systemic lupus erythematosus and pregnancy confers an increased rate of fetal death "in utero" and spontaneous abortion (miscarriage), as well as of neonatal lupus.
Behçet's disease.
Pregnancy does not have an adverse effect on the course of Behçet's disease and may possibly ameliorate its course. Still, there is a substantial variability in clinical course between patients and even for different pregnancies in the same patient. Also, the other way around, Behçet's disease confers an increased risk of pregnancy complications, miscarriage and Cesarean section.
Multiple sclerosis.
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.
Others.
The following conditions may also become worse or be a potential risk to the pregnancy: