Answer from Dr. Locker
Penis size should not be an issue for conception. Regardless of size, the penis can ejaculate sperm with enough force to make the sperm enter the cervix and travel into the woman's reproductive system to potentially fertilize her egg. Sexual position, however, does matter.
The sperm should be deposited so that it moves toward the back of the vagina, to go into the cervix. Rather than having sex with the woman on top (which could cause semen to leak out after ejaculation) instead, the woman could be more likely to get pregnant if they have sex with the man on top. She should be on the bottom, with her legs up or wrapped around him. In fact, she should try to keep her hips tilted a bit up (perhaps with a pillow under her butt). Then after sex, she should stay on her back with her legs up in the air (or knees bent up to her chest)so that the semen does not run out right away. The goal should be for the woman to keep the man's semen inside her during sex, and when he withdraws his penis, and also for a while after sex. After he ejaculates and carefully withdraws his penis, for 15 to 30 minutes she should keep her legs up so that the sperm is encouraged to move into her reproductive system.
Please talk with your gynecologist about your questions and concerns about conception. Your doctor will give you more information, as well as give you a pre-conception exam and prescribe prenatal vitamins to you (which you should start taking as soon as you are trying to get pregnant). Your doctor can also talk with you about your monthly cycle, to help you determine the days when you are most likely to be ovulating, and, therefore, the days you are most likely to get pregnant. Also, trying using Ovulation Predictor tests, which you can buy at any drug store to figure out when you are going to be ovulating (and read the directions in the box to learn more about when to have sex). Also, for more details about how to get pregnant, check out a book called: Taking Charge of Your Fertility: The Definitive Guide to Natural Birth Control, Pregnancy Achievement, and Reproductive Health by Toni Weschler.
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