Study of the Safety and Contraceptive Efficacy of C31G Compared to Conceptrol®

Inclusion Criteria: Potential subjects must: * Be healthy women, who are sexually active, at risk for pregnancy and desiring contraception. * Be within the age range of 18 through 40 years inclusive. * Be at low-risk for human immunodeficiency virus (HIV) or sexually transmitted disease (STD) infection and currently have a single sex partner (minimum 4 months) who is also at low-risk for HIV or STD. * Have a negative urine pregnancy test prior to enrollment. * Have normal cyclic menses with a usual length of 24 to 35 days over the last 2 cycles or at least one spontaneous, normal menstrual cycle (2 menses) since delivery, abortion, or after discontinuing hormonal contraception/hormonal therapy. * Be willing to accept a risk of pregnancy. * Be willing to engage in at least 4 acts of heterosexual vaginal intercourse per month for a period of 6 months. * Be willing to be randomized to either study treatment. * Be willing to only use the study product as the primary method of contraception over the course of the study with the exception of emergency contraception (EC), when indicated. * Be capable of using the study product properly and agree to observe all study directions and requirements. * Be willing to keep a diary to record coital information, product use information, information about the use of other vaginal products, and sign and symptom data of subject and partner. * Agree not to participate in any other clinical trials during the course of the study. * Be willing to give written informed consent to participate in the trial. Exclusion Criteria: Potential subjects must not: * Have a history of allergy or sensitivity to spermicides or products containing N-9. * Have had 3 or more urinary tract infections (UTI) in the past year. * Have UTI by urine culture or symptomatic yeast vaginitis or symptomatic bacterial vaginosis diagnosed by wet mount unless treated and proof of cure is documented. * Be pregnant, have a suspected pregnancy or desire to become pregnant during the course of the study. * Have a history of infertility or of conditions that may lead to infertility, without subsequent intrauterine pregnancy. * Have any contraindications to pregnancy (medical condition) or chronic use of category D or X medications. * Have had more than one sexual partner in the last 4 months. * Have shared injection drug needles within the past 12 months. * Have or have been suspected to have HIV infection. * Have been diagnosed with genital herpes simplex virus (HSV), with the first occurrence (initial episode) within three months prior to screening. * Have 3 or more outbreaks of HSV within the last year. * Have been diagnosed with any other STDs (including trichomonas) in the 6 months prior to the screening visit (with the exception of Human Papilloma Virus \[HPV\]). * Be lactating or breastfeeding. * Have any clinically significant abnormal vaginal bleeding or spotting within the month prior to screening. * Have any clinically significant abnormal finding on pelvic examination or baseline labs, which in the view of the investigator, precludes her from participating in the trial. * Have clinically significant signs of vaginal or cervical irritation on pelvic examination. * Have had vaginal or cervical biopsy or vaginal surgery within 3 months prior to screening. * Have used vaginal or systemic antibiotics or antifungals within 14 days prior to screening or randomization. * Have had a Depo-Provera® injection in the 10 months prior to enrollment. * Have an abnormal Pap smear with high grade squamous intraepithelial lesion (HSIL), atypical glandular cells (AGC) or ASC-H (atypical squamous cells, cannot exclude HSIL) within the last 12 months. * Have an abnormal Pap smear with low-grade squamous intraepithelial lesion (LSIL) or ASCUS-HPV HR positive unless resolved by colposcopy. * Have a Cervical Intraepithelial Neoplasia (CIN) diagnosis by biopsy within the last 12 months. * Have a history or a current diagnosis of cervical cancer. * Consume (on average) greater than three drinks of an alcoholic beverage per day. * Have a past history (within twelve months) or current history of drug abuse \[recreational, prescription or over-the-counter (OTC)\]. * Have taken an investigational drug or used an investigational device within the past 30 days. * Have previously participated in or completed this study or any other phase III study of C31G. * Have issues or concerns (in the judgment of the investigator) that may compromise the safety of the subject or confound the reliability of compliance and information acquired in this study.