We receive hundreds of messages daily from women who experience this situation. This is a nightmare for many women, as many of them are not able to get pregnant.

Back flow of semen following intercourse may be caused by several factors. 

1. Mostly due to liquefaction of semen (change in consistency) with time after ejaculation. 

2. Partner's large amount of ejaculation following abstinence for few days

3. More watery ejaculation state 

4. Increased cervical or vaginal secretion following your highly aroused state 

5. Increased vaginal temperature 

Flow-back from these causes may not impact on your chances of conception. You need not to be worried regarding this. Just enjoy your normal sex life.

You can opt for some measures like lifting your hips or taking a pillow behind buttock after his ejaculation which may prevent back flow to some extent. Maintain your genital hygiene before and after each sexual activity.

HOWEVER THERE ARE OTHER CAUSES THAT MAYBE MORE PROBLEMATIC.

1. Antisperm antibody
The man or the woman (or both) may produce antisperm antibody. This is a chemical substance produced by the immune system which misrecognises the sperm as an enemy and goes all out to attack. If antisperm antibodies come from the man, the tails of the sperm cells may be attacked, rendering them immobile. Should the antibodies come from the woman, the heads of the sperm cells are likely targets. In either case, the sperm cells become weakened and are unable to penetrate the mucus secretion of the cervix, which sits on the vagina and forms the gateway to the womb. 
In the same vein, if the woman’s vagina is too acidic, it becomes inhospitable to the sperm cells which thrive in an alkaline environment and may thus inadvertently kill them. Antisperm antibodies in either the man or woman suggest a previous INFECTION. 

2. Defective cervical mucus
One of the functions of cervical mucus is to help guide or transport the sperm to the egg or ovum. The mucus is thin, watery and stretchy before and during ovulation. But after ovulation, consistency of the mucus changes and it becomes thick and sticky, inelastic and inhibitive to the movement of sperm, closing the way to the cervix and to the womb and the egg. With the gate thus shut, the sperm naturally flows backward and outwards! Thick cervical mucus all-cycle-round may indicate inflammation and excitement of the glands of the cervix. This may be due to an infection, diary intolerance, irritation by chemical agent in soaps, tampons or sanitary pads and douching substances. Hormonal imbalance may also have a hand in it. So also over consumption of white sugar, and overuse of medications which may have upset the balance of friendly bacteria population vis¬a-vis the unfriendly bacterial flora.

3. ORGASM. Before a woman attains orgasm, there is an increased blood flow to the genital area in preparation for the receipt of the sperm. Orgasm disperses the blood, thus decongesting the genital tract to enable the sperm move more swiftly. The contraction of the muscles as well helps to transport the sperm. Where a woman does not attain orgasm before the sperm are received, this could harm her conception, although this is not a cast iron rule. 

4. Retroverted uterus. In women with retroverted uterus, the mouth of the uterus sitting on the vagina is pushed forward. This means sperm may first be fired to the back side of the cervix from where they have to find their way back again to the neck of the cervix. Many never make it. The solution is to lie on her right-hand or left-hand side whenever you and your husband are trying to conceive. What side to lie on will depend on to which side the uterus is tilted. Discuss this with your gynaecologist. 

5. PENIS that is too long. It is believed that men with too long penis may misfire sperm during ejaculation and miss the target. For this reason, some authorities suggest that they pull back the shaft about three inches in the vagina vault just before ejaculation.