The aim of our study is to evaluate the feasibility, and diagnostic value of oral 13C labelled glucose breath test compared to the GCT and OGTT tests.

This prospective observational pilot study will include pregnant patients with a singleton fetus up to 33 weeks gestation.

Patients meeting the inclusion criteria who are willing to participate will be asked to drink 50 or 100 grams of glucose in 200 ml water (disposable plastic cup). The test will start at approximately 08:00 with no change in normal diet is needed prior to the GCT, or after an 8-14 hour overnight fast for the OGTT.

The test will be performed at the Maternal Fetal Medicine outpatient clinics, the Maternal Fetal Medicine unit for inpatients or at the patient's residence.

Samples collection

Glucose samples:

1. Venous blood samples to measure serum glucose levels will be drawn at a timing according to the performed test: for GCT at one hour after the ingestion of glucose (standard 50 grams GCT) and for OGTT at to, t1, t2 and t3 (standard 100 grams OGTT).
2. Finger prick glucometer (DEX) readings will be done at one and two hours after the ingestion of glucose for participants completing GCT.

Each participant will receive a kit containing 30 12ml exetainers (Labco, UK), a compatible strew and a stamped envelope (addressed to OBGYN department, Kaplan Medical Center, Rehovot). The exetainers will be numbered sequentially and marked by the participant code (initials and participant number). After a short explanation on how to use the kit participants will be asked to sample their breath every 15 minutes starting 30 minutes before the ingestion of glucose and ending six hours after the ingestion. Patients who are taking the test at the Maternal Fetal Medicine clinic will be asked to remain at the clinic for the duration of six hours, until completing all of the samples. Patients who are taking the test while admitted to the Maternal Fetal Medicine ward and are due for discharge will not be delayed as discharge is usually until 14:00. During the six hours of participation patients will be ask to avoid food or drink. Afterwards, participants will keep the envelope in their home until a pickup is coordinated by a member of the research team.

Samples can be kept up to three months before being processed. The coded samples will next be sent to Amir Shafat's lab at Ireland for processing. Samples will be analyzed on a continuous-flow Isotope Ratio Mass Spectrometer (ABCA, Serco, UK) for breath CO2 enrichment against Vienna-Pee Dee Belemnite, a known international standard for 13C. Enrichment will be expressed in atom fraction and δV-PDB. Pilot data in Amir's lab demonstrated that this procedure gives excellent resolution of breath enrichment (to within 1 part per million) and includes peak excretion of 13CO2 in all volunteers. Percent dose recovered and other parameters indicating the rate of glucose oxidation will be calculated from the enrichment curve. Amir's laboratory has extensive experience in stable isotope and breath testing.

A few days (and no longer than one week) after the test participants will receive their GCT or OGTT result by phone and fax/email. Participants will be asked to present the test result to their physician in the next prenatal visit. For participants with a pathological GCT score, >139 mg/dL, a letter will be sent to the attending physician requesting to perform the 100 grams OGTT either as part of the study or at the health care provider clinic. Participants with one or more elevated glucose values in the OGTT, will be asked to contact their physician as soon as possible for further guidance. Two weeks after the test, a follow-up phone call will be done to all participants with a pathological result in order to ensure that the OGTT had been completed and to record the OGTT result in case of a pathological GCT, or to ensure the appropriate surveillance has begun in case of a pathological OGTT.

Demographic data including: a) maternal age; b) maternal height: c) pre-pregnancy and current weight; d) obstetrical history e) background medical illnesses and medications.

Sample size estimation In this pilot study the investigators aim to recruit 50 participants in each group (GCT and OGTT) out of which it is expected that 8-11 will have pathological result (16-21% pathological GCT).

Inclusion Criteria:

• Pregnant women
• ages 18 to 45 years old
• A singleton pregnancy
• Confirmed gestational age by a documented first-trimester ultrasound.

Exclusion Criteria:

• Patients below the age of 18 or above 45 years-old
• Patients diagnosed with gestational or pre-gestational diabetes
• Patients with multiple pregnancies or fetuses with congenital and/or chromosomal anomalies
• Patients after a multifetal pregnancy reduction (spontaneous or iatrogenic)
• Patients not having a sonographic dating during the first trimester of pregnancy
• Patients with conditions or medications that affect glucose metabolism
• Patients who will refuse to consent, or are not fit or able to read, understand, or sign an informed consent.

This prospective observational pilot study will include pregnant patients with a singleton fetus up to 33 weeks gestation.