Managing Pre-Existing Conditions (Diabetes, Thyroid, BP) Before Pregnancy

If you're living with diabetes, thyroid disease or chronic hypertension and planning a pregnancy, the work that happens before conception matters as much as anything that happens after. This guide covers the specific glucose targets, medication switches and blood pressure thresholds that current evidence-based guidelines recommend getting in place before you try to conceive, and pairs with our pre and post pregnancy services.

This article is for general educational purposes only and does not replace personalised medical advice. Medication changes for diabetes, thyroid disease or hypertension should always be made with your prescribing provider - never stop or switch a medication on your own.

In short: before conceiving, aim for an HbA1c under 6.5%, a TSH under 2.5 mIU/L and blood pressure under 140/90 mmHg. Switch off ACE inhibitors, ARBs, statins and SGLT2 inhibitors before stopping contraception, and complete GLP-1 medication washout (1 to 8 weeks, depending on the drug) before trying to conceive.

• Diabetic embryopathy risk rises in direct proportion to early HbA1c - the preconception target is under 6.5%, tightening to 6-6.5% once pregnant.
• GLP-1 medications need a drug-specific washout before conception: 8 weeks for semaglutide, 4-8 weeks for tirzepatide, 3 days to 4 weeks for liraglutide, and 12 weeks for extended-release exenatide.
• The CHAP trial changed hypertension management: treating mild chronic hypertension (140/90 to 159/109 mmHg) now starts at 140/90 mmHg, not only at severe levels of 160/110 mmHg and above.
• Low-dose aspirin for preeclampsia prevention differs by guideline: ACOG recommends 81 mg/day, while FIGO and ISSHP recommend 150 mg at bedtime - the higher dose shows stronger platelet suppression.
• Methimazole must be switched to propylthiouracil before conception because of a specific embryopathy risk during weeks 6 to 10 of gestation.

Preconception optimisation for chronic conditions means bringing diabetes, thyroid function and blood pressure into tightly defined target ranges, and switching off medications that are unsafe in pregnancy, before conception happens. Major fetal organ development takes place in the first 8 to 10 weeks - often before a pregnancy is confirmed.

Diabetic embryopathy - malformations such as congenital heart defects, neural tube defects or renal agenesis - occurs in direct proportion to how high maternal blood sugar runs during early organogenesis. Because the fetal thyroid doesn't function until 12 to 14 weeks, an embryo relies entirely on maternal thyroid hormone crossing the placenta during the first trimester, which means uncontrolled maternal hypothyroidism directly limits fetal neurodevelopment during that stretch. And with chronic hypertension complicating 2-5% of pregnancies, uncontrolled blood pressure raises the risk of superimposed preeclampsia, placental abruption and fetal growth restriction.

All three conditions share the same underlying lesson: control achieved before conception protects a developmental window that closes before most people know they're pregnant. These are also the conditions that most often lead to high-risk pregnancy care later on.

Insulin (regular, NPH, lispro, aspart) remains the gold standard during preconception and pregnancy because it doesn't meaningfully cross the placenta. Metformin and glyburide do cross the placenta - metformin is sometimes continued during PCOS ovulation induction, but current guidance recommends transitioning to insulin at preconception or immediately on a positive test.

GLP-1 receptor agonists (semaglutide, liraglutide, tirzepatide) restore fertility in people with obesity or PCOS-related subfertility, which has led to a rise in unplanned pregnancies during active treatment. Animal studies show delayed fetal development and skeletal changes at high exposures, so full washout before conception is standard practice.

During washout, expect appetite and weight rebound. Your provider will typically transition you to basal-bolus insulin or optimised metformin to maintain glycemic control while you're off the GLP-1.

Hypothyroidism. Target a TSH under 2.5 mIU/L before conception. Once pregnant, levothyroxine requirements typically rise 20-30%, and many patients are counselled to add two extra weekly doses (roughly a 30% increase) as soon as they get a positive home test, pending formal labs every four weeks through the first half of pregnancy.

Hyperthyroidism. Methimazole exposure during weeks 6 to 10 of gestation is linked to specific embryopathies - aplasia cutis congenita, choanal atresia, esophageal atresia and dysmorphic facial features. Because of this, anyone on methimazole who is planning pregnancy should switch to propylthiouracil before stopping contraception. Propylthiouracil carries its own rare risk of severe liver toxicity, so patients typically switch back to methimazole around 16 weeks, at the start of the second trimester, for the rest of pregnancy and lactation.

For years, guidelines avoided treating mild chronic hypertension (systolic 140-159 mmHg or diastolic 90-109 mmHg) until blood pressure reached severe levels of 160/110 mmHg or higher, out of concern that lowering blood pressure might reduce placental blood flow and cause growth restriction. The Chronic Hypertension and Pregnancy (CHAP) trial overturned that assumption: treating mild chronic hypertension to a target under 140/90 mmHg significantly reduced severe preeclampsia, medically indicated preterm birth before 35 weeks, placental abruption and fetal or neonatal death, without increasing small-for-gestational-age births. Following CHAP, ACOG and SMFM updated guidance to start treatment at 140/90 mmHg rather than waiting for severe hypertension.

Aspirin dosing data tells a similarly evidence-driven story. The ASPRE trial found that 150 mg of aspirin nightly reduced preterm preeclampsia (before 34 weeks) by 62% and early-onset preeclampsia (before 37 weeks) by 82% in high-risk pregnancies, with 150 mg producing 95.4% thromboxane B2 suppression versus 79.1% at the lower 81 mg dose. That may explain why higher-dose protocols outperform in patients with elevated BMI or baseline inflammation.

Pregnancy-safe blood pressure dosing. Labetalol 100 mg twice daily, up to 2,400 mg/day in divided doses; extended-release nifedipine 30-90 mg once daily; methyldopa, which has a long safety record though sedation limits its use for some people.

Aspirin protocols compared.

Folate and iodine. Everyone with diabetes who is planning pregnancy should take a prenatal vitamin with 400-800 mcg folic acid and 150 mcg potassium iodide. Standard synthetic folic acid at 400 mcg/day protects against neural tube defects regardless of MTHFR genotype, though people with confirmed MTHFR variants may be counselled toward bioavailable methylfolate instead.

• Diabetes: dilated eye exam preconception or in early first trimester, urine albumin-to-creatinine ratio and eGFR, and an ankle-brachial index if diabetes duration exceeds 10 years or cardiovascular risk is elevated.
• Thyroid: confirm TSH under 2.5 mIU/L, and switch methimazole to propylthiouracil if applicable.
• Blood pressure: confirm control under 140/90 mmHg, and switch off ACE inhibitors, ARBs, mineralocorticoid antagonists, statins and SGLT2 inhibitors.
• Everyone: start or confirm a prenatal vitamin with adequate folic acid and iodine - this pairs naturally with broader preconception nutrition and vaccine planning.
• Coordinate specialties: endocrinology, cardiology and obstetrics (or maternal-fetal medicine if risk factors stack) should all be looped in before contraception stops.

Managing diabetes, thyroid disease or chronic hypertension before pregnancy isn't just about staying healthy in general - it's about hitting specific, evidence-based targets and completing medication transitions before the developmental window that these conditions most affect. Coordinating with your endocrinologist, cardiologist and obstetric provider before you stop contraception gives every one of these steps time to actually work.

If you're currently pregnant and taking a medication flagged above as unsafe - an ACE inhibitor, an ARB, methimazole or a GLP-1 medication - contact your provider promptly rather than stopping abruptly on your own. Timing and alternatives matter, and your care team can guide the safest next step.

• ADA Standards of Care - Management of Diabetes in Pregnancy.
• PMC - GLP-1 Receptor Agonists and Preconception Planning.
• American Thyroid Association - Hypothyroidism in Pregnancy.
• ACC - Chronic Hypertension and Pregnancy (CHAP) Trial.
• PMC - Controversies and Clarifications on Aspirin in Preeclampsia Prevention.
• CDC - MTHFR Gene Variant and Folic Acid Facts.