Insulin and Blood Sugar in Pregnancy Planning: What You Need to Know Before You Conceive

Why Blood Sugar Control Before Conception Matters

Achieving good blood sugar control before conception is the single most effective intervention for reducing diabetes-related pregnancy complications. The American Diabetes Association (ADA) 2024 Standards of Care state that preconception counseling should be offered to all women of reproductive age with diabetes, and that an HbA1c below 6.5% before conception is associated with the lowest risk of congenital anomalies [1]. Organogenesis, the period during which the baby's heart, brain, and spine form, occurs between weeks 3 and 8 of pregnancy, often before a woman even knows she is pregnant. Elevated glucose during that window is directly toxic to developing tissues.

A 2014 meta-analysis published in Diabetologia (N=5,760 pregnancies across 28 studies) found that every 1% rise in periconceptional HbA1c above 6.3% was associated with an approximate 30% increase in the rate of major congenital malformations [2]. Spontaneous miscarriage rates are also elevated; women with HbA1c above 10% face roughly a twofold higher early pregnancy loss rate compared with those who enter pregnancy with HbA1c below 7% [3].

Insulin is the only glucose-lowering agent with a long safety record across all trimesters. Human insulin and several analogs do not cross the placenta in clinically meaningful amounts, making them the preferred pharmacological tool from preconception through delivery [4].

Preconception HbA1c Targets and How Long It Takes to Get There

Most clinicians and guidelines recommend reaching a stable HbA1c below 6.5% for at least three consecutive months before attempting conception. NICE guideline NG3 (updated 2023) specifies this threshold while acknowledging that targets must be individualized to avoid frequent or severe hypoglycemia, which carries its own fetal risks [5].

Red blood cells have a lifespan of roughly 90 days, which is why HbA1c reflects average glucose over the preceding three months. A woman starting preconception optimization with an HbA1c of 9% should realistically plan a minimum six-month runway before conception if aggressive titration is pursued. Monthly glucose monitoring paired with three-monthly HbA1c checks is a reasonable schedule during this phase.

Basal-bolus insulin regimens are frequently required for type 1 diabetes (T1D) and advanced type 2 diabetes (T2D). A continuous glucose monitor (CGM) accelerates target attainment; the CONCEPTT trial (N=325 pregnant women with T1D) demonstrated that CGM use during pregnancy reduced the rate of large-for-gestational-age infants from 69% to 53% (P<0.001) and cut neonatal hypoglycemia requiring intravenous glucose by roughly 24% [6]. Starting CGM before conception is therefore strongly encouraged by the ADA and the Endocrine Society [1][7].

Which Medications Are Safe to Continue, and Which Must Stop

Not every drug a woman with diabetes takes can safely continue into pregnancy. The following framework guides prescribers.

Insulin analogs. Insulin lispro (Humalog) and insulin aspart (NovoLog) have pregnancy Category B data and are widely used. A randomized trial by Mathiesen et al. (N=322) found that insulin aspart during pregnancy produced equivalent glycemic control to human regular insulin with a lower rate of nocturnal hypoglycemia [8]. NPH remains the basal insulin with the most long-term safety data in pregnancy, though insulin detemir also carries Category B labeling based on the PROTECT trial [9].

Metformin. Metformin crosses the placenta. The MiG trial (N=751) compared metformin with insulin in gestational diabetes and found similar neonatal outcomes, though metformin-treated women had higher rates of supplemental insulin use (46%) [10]. The ADA notes metformin may be continued for polycystic ovary syndrome (PCOS) management into early pregnancy to reduce miscarriage risk, but insulin should be added for glucose control once gestational diabetes is diagnosed [1].

GLP-1 receptor agonists. Semaglutide (Ozempic, Wegovy), liraglutide (Victoza), and all other GLP-1 agonists should be stopped at least two months before conception. Animal studies showed fetal harm at clinical doses, and adequate human safety data do not exist [11]. The FDA label for semaglutide explicitly states it should not be used in pregnancy [12].

SGLT2 inhibitors. Canagliflozin, empagliflozin, and dapagliflozin must be stopped before conception. Rodent studies showed renal developmental toxicity, and the FDA issued a label update warning against use in the second and third trimesters [13].

ACE inhibitors and ARBs. These are commonly prescribed for diabetic nephropathy. Both classes are fetotoxic from the second trimester onward and should be switched to a pregnancy-safe antihypertensive (labetalol, nifedipine, or methyldopa) before conception [14].

Statins. All statins are contraindicated in pregnancy and should be stopped before conception [14].

Blood Glucose Targets During Pregnancy

Pregnancy itself lowers fasting glucose and blunts postprandial peaks in women without diabetes, so the targets used in non-pregnant adults are inappropriate. NICE NG3 and the ADA 2024 Standards both specify tighter ranges [1][5]:

• Fasting: 3.5 to 5.3 mmol/L (63 to 95 mg/dL)
• One hour after meals: below 7.8 mmol/L (140 mg/dL)
• Two hours after meals: below 6.4 mmol/L (115 mg/dL)

These targets are tighter than standard T2D management goals because maternal hyperglycemia drives fetal hyperinsulinism, macrosomia, and the downstream risks of shoulder dystocia, Caesarean delivery, and neonatal hypoglycemia. A 2009 Cochrane review of tight versus less-tight glucose control in gestational diabetes (N=1,000) found that tight control reduced macrosomia rates from 22% to 10% without increasing maternal hypoglycemia events significantly [15].

Hypoglycemia is equally dangerous. Maternal hypoglycemia below 3.0 mmol/L (54 mg/dL) triggers stress hormone release that can compromise uteroplacental blood flow. Women using intensive insulin regimens should keep a fast-acting glucose source within arm's reach and should set CGM low alerts no lower than 3.9 mmol/L (70 mg/dL) during pregnancy.

Insulin and Blood Sugar in Special Populations

In Older Adults Planning a Later Pregnancy

Women conceiving after age 35 carry a higher baseline rate of gestational diabetes (roughly 13% at age 35 to 39 versus 6-9% overall) and are more likely to enter pregnancy with established T2D or impaired fasting glucose [16]. Insulin resistance increases with age independently of body weight because of declining adiponectin levels and age-related changes in skeletal muscle glucose uptake. Preconception evaluation for these women should include a fasting glucose, two-hour oral glucose tolerance test (OGTT), HbA1c, renal function panel, and a lipid profile, even if no prior diabetes diagnosis exists. An HbA1c of 5.7% to 6.4% in a 38-year-old woman planning pregnancy warrants early dietitian referral and, in many cases, preemptive insulin sensitization counseling before conception rather than a wait-and-see approach.

The Endocrine Society's 2013 guideline on management of diabetes in pregnancy (reaffirmed 2023) recommends that older women with prediabetes undergo a 75 g OGTT at the first prenatal visit rather than waiting for the standard 24-to-28-week screen [7].

In Children and Adolescents With Type 1 Diabetes

Adolescents with T1D who become sexually active need preconception counseling as early as their mid-teens, because half of all pregnancies in the U.S. are unplanned. The ADA notes that only 34% of reproductive-age women with T1D receive any documented preconception counseling [1]. An unplanned pregnancy with a T1D teenager often begins with an HbA1c above 8%, placing the embryo in a high-risk teratogenic environment during the first eight weeks.

Insulin dose requirements change dramatically across the female menstrual cycle. Estrogen and progesterone both increase insulin resistance during the luteal phase; daily insulin needs may rise 10 to 20% in the two weeks before menstruation. Girls with T1D should be coached to track cycle phase alongside CGM data so they can anticipate these swings. The SEARCH for Diabetes in Youth study found that only 21% of adolescent girls with T1D had HbA1c below 7.5%, the threshold above which preconception risk escalates sharply [17].

Contraceptive counseling is part of preconception planning for this group. Low-dose combined oral contraceptives have a minimal effect on glycemic control in most adolescents with T1D, making them an appropriate bridge while HbA1c is being optimized [18].

In Adults With Renal Disease

Diabetic nephropathy complicates roughly 20 to 40% of long-standing T1D and T2D cases, and renal function has a direct bearing on pregnancy safety and on insulin pharmacokinetics. The kidney is a major site of insulin degradation; as glomerular filtration rate (GFR) falls, insulin clearance slows, meaning the same dose produces a longer and larger hypoglycemic effect. Women with a GFR below 60 mL/min/1.73 m² may require insulin dose reductions of 25 to 50% compared with their pre-renal-disease requirements [19].

Pregnancy itself causes a physiological 40 to 50% increase in GFR during the first two trimesters, which can temporarily mask the severity of underlying renal disease. In women with a baseline GFR below 40 mL/min/1.73 m², this hyperfiltration-driven masking gives way to an accelerated decline in kidney function that may not fully reverse postpartum. A 2015 study in the Journal of the American Society of Nephrology (N=504) found that women with a pre-pregnancy GFR below 40 mL/min/1.73 m² had a 43% rate of pregnancy-related permanent renal function decline [20].

Metformin is contraindicated when eGFR falls below 30 mL/min/1.73 m² due to lactic acidosis risk, leaving insulin as the only oral-adjacent option. SGLT2 inhibitors lose efficacy below eGFR 45 and should not be used in preconception or pregnancy planning in this group regardless. Nephrologist co-management is strongly recommended for any woman with eGFR below 60 who is planning conception.

ACE inhibitor and ARB discontinuation presents a particular challenge in this population because these drugs slow the progression of diabetic nephropathy. The ACOG Committee Opinion No. 723 recommends switching to labetalol or nifedipine for blood pressure management as soon as pregnancy is planned, accepting a possible short-term worsening of proteinuria as the trade-off for fetal safety [14].

Folic Acid, Thyroid, and Other Preconception Labs

Women with diabetes need a higher folic acid dose than the general population. Standard guidance recommends 400 mcg daily for most women, but the CDC and NICE NG3 both recommend 5 mg (5 to 000 mcg) daily for women with pre-existing diabetes, starting at least three months before conception and continuing through the first trimester, because neural tube defect risk is elevated in hyperglycemic pregnancies [5][21].

Thyroid dysfunction is common in women with T1D; autoimmune thyroid disease (Hashimoto thyroiditis) occurs in approximately 17 to 30% of T1D patients [22]. An unrecognized TSH above 2.5 mIU/L at the time of conception is associated with a higher miscarriage rate. The ATA/AES 2017 guideline on thyroid disease in pregnancy recommends checking TSH in all women with T1D or autoimmune disease who are planning pregnancy [23].

Additional baseline labs before conception in a diabetic woman should include:

• HbA1c (target below 6.5%)
• Fasting glucose and two-hour OGTT if HbA1c is in the prediabetes range
• Serum creatinine and cystatin-C-based eGFR
• Urine albumin-to-creatinine ratio
• TSH with reflex free T4
• Complete lipid panel
• Retinal exam (diabetic retinopathy can worsen in pregnancy)
• Blood pressure measurement and 24-hour urine protein if microalbuminuria is present

The retinal exam deserves particular attention. Diabetic retinopathy is known to worsen during pregnancy, especially if glycemic control improves very rapidly from a high baseline. The Diabetes Control and Complications Trial (DCCT) showed that the initial phase of intensive glycemic improvement was paradoxically associated with a transient worsening of retinopathy in 13.1% of patients [24]. A preconception retinal baseline allows ophthalmologists to distinguish pregnancy-related worsening from coincidental progression.

Gestational Diabetes: Screening, Diagnosis, and Insulin Initiation

Women without pre-existing diabetes are screened for gestational diabetes mellitus (GDM) at 24 to 28 weeks of gestation using either a one-step 75 g OGTT or the two-step 50 g glucose challenge followed by a 100 g OGTT [1]. The International Association of Diabetes and Pregnancy Study Groups (IADPSG) one-step criteria (fasting glucose at or above 5.1 mmol/L, one-hour at or above 10.0 mmol/L, or two-hour at or above 8.5 mmol/L) are used in many health systems and by the ADA [1][25].

GDM affects 6 to 9% of all U.S. pregnancies, per CDC estimates [21]. Women with a prior GDM pregnancy have a 33 to 50% recurrence risk in subsequent pregnancies [26]. Risk factors include BMI above 30 kg/m², prior macrosomic infant (birthweight above 4 kg), PCOS, first-degree relative with T2D, and non-European ethnicity.

First-line treatment is medical nutrition therapy (MNT) targeting the glucose goals listed above. When MNT fails after one to two weeks, insulin is initiated. A typical starting regimen uses NPH at bedtime for fasting hyperglycemia (0.1 to 0.2 units/kg) and rapid-acting insulin aspart or lispro before the largest meal for postprandial spikes. Dose escalation follows a structured titration: fasting glucose checks drive basal adjustment by 2 units every three days until the fasting target is met [1].

Intrapartum and Postpartum Blood Sugar Management

Labor and delivery require careful glucose management because maternal hyperglycemia during labor drives neonatal hypoglycemia in the first hours of life. Most labor and delivery units target maternal glucose between 4.0 and 7.0 mmol/L (72 to 126 mg/dL) during active labor using variable-rate intravenous insulin infusions or subcutaneous rapid-acting insulin on an hourly basis [5].

After delivery, insulin resistance drops sharply because the placenta, a major source of counter-regulatory hormones including human placental lactogen, is expelled. Women with T1D often need to reduce their insulin dose by 30 to 50% immediately postpartum. Women who had GDM generally see their insulin requirements disappear within 24 to 48 hours of delivery.

Postpartum follow-up for women with GDM should include a 75 g OGTT at six to twelve weeks postpartum. The ADA reports that only 50 to 60% of women with GDM receive this test, despite the fact that 5 to 10% will be found to have overt T2D and another 30 to 40% will have impaired glucose tolerance at that visit [1][27]. Women who breastfeed have a modestly lower short-term risk of progressing to T2D; the SWIFT study (N=1,010) found that exclusive breastfeeding for at least two months reduced the two-year incidence of T2D by 46% in women with prior GDM (P<0.01) [28].

Metformin can be reintroduced postpartum for women who are at high risk of T2D conversion. The Diabetes Prevention Program (N=3,234) demonstrated that metformin 850 mg twice daily reduced T2D incidence by 31% over 2.8 years compared with placebo in high-risk adults, with the greatest benefit seen in women with a history of GDM [29].

"For women with a history of gestational diabetes, postpartum glucose testing and lifestyle counseling should be viewed as an ongoing preventive care imperative, not a one-time postpartum box to check," states the ADA 2024 Standards of Care, Section 15 [1].