The investigators want to learn how the body's response to insulin changes during pregnancy in people with Type 1 Diabetes.

In this study, the investigators will look at information from people who used a Dexcom continuous glucose monitor (CGM) and a Tandem insulin pump while they were pregnant. Participants will fill out an online survey and agree to share their device data. The investigators will use this information to see how insulin sensitivity changes from three months before pregnancy to six months after the baby is born.

The aim of this retrospective observational study is to collect continuous glucose monitor (CGM) and insulin pump data from pregnant individuals with Type 1 Diabetes, from 3 months before conception to 6 months after delivery. This data will be used to estimate how insulin sensitivity (IS) changes across gestation, with the goal of better understanding glucose-insulin dynamics during pregnancy and develop algorithms for adapting insulin administration across gestational ages.

Participants will complete an online survey where they will provide their insulin pump serial number, date of delivery, gestational week at delivery, date of last menstrual cycle (if known), and other optional demographic information. The study team will use this information to retrieve participants' CGM and insulin infusion data from 3 months before conception to 6 months after delivery.

Up to 200 participants will be screened, with the goal of reaching 100 screened participants.

Inclusion Criteria:

• Age >=18 years old
• Pregnant within the last 8 years
• Diagnosed with type 1 diabetes for at least one year at the time of pregnancy
• Use of a Dexcom CGM system (Dexcom, Inc., CA) and Tandem insulin infusion pump (Tandem Diabetes Care, CA) across gestation, and willing to share data from this pregnancy with study staff.

Adult individuals with T1D who received pregnancy care within the last 8 years in one of the participating sites: UVA Health (Charlottesville, VA), Northwestern Memorial Hospital (Chicago, IL).