Sperm cells are either X or Y. The Egg cell is always X. If X-Sperm cell meets the egg(X) the baby will be a Girl (XX). If Y-Sperm cell meets the egg(X), the baby will be a Boy (XY). The Y-Sperm cells move fast but die fast (its lifespan is about 24hrs). X-Sperm cells move slowly but survive longer (up to 3-4days).
Therefore, if you have intercourse some days before ovulation, most of the Y-Sperm cells would have died leaving mostly X-Sperm cells to fertilized the egg
Here's a simple way to calculate your ovulation period.
1. From the first day you see the blood of your period.
2. Pick up a calendar and count 15 days including the first day of the blood stain.
3. Mark the 15th day with a pen.
4. Mark 3 days before the 15th day, and mark 3 days after the 15th day.
6. You will notice you have 7 days marked.
These 7 days are your ovulation/fertile days also known as unsafe period.
That is if you have sex on any of these days, then you have 98% chances of being pregnant.
7. Do this every month diligently
For Example
1. If your period is 11th July, ,
2. 15 days after the first day of the blood stain will be on 25th of this July too (2weeks after) counting from 11th.
3. 15th day is on the 25th July
4. 3days before the 15th day is 22, 23, 24.
5. 3days after 25th July is 26,27,28.
6. 22-28th July (7days) is when you are ovulating. It is your fertile period.
**Don't forget that the 4 days is a reminder that sperm stays in the female body for about 4 days. Any sexual contact in between ovulation period will form something ...
If you want a baby girl, have
sex between 3 days before ovulation (22- 24th)
Female sperm swim slowly but can stay inside woman body for days without dying off,
For a baby boy on the main ovulation day and 3 days after 25th.(26,27,28)
Male sperm swim very fast but will get weak and die off if no egg found for a while leaving female sperm to finish the job .
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MAJOR SIGNS THAT YOUR OVULATION HAS BEGAN...
You don't need a doctor, just relax. After 2 weeks you may feel the following:
1. Headache.
2. High body temperature.
3. Dizziness (feeling of tiredness and sleepiness).
4. Breast tenderness/ size increases just like when you are menstruating.
5. Increased sex drive.
6. Light cramps or pain on one side of the pelvis.
7. Abdominal bloating.
8. Heightened sense of smell, taste or vision (you eat more).
9. Vagina mucus becomes slippery (if you notice, put it in between fingers...it stretches like thick catarrh or snail mucus...colorless like egg white.
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N/B: The instance here is for ladies with 30 days cycle. And someone who saw her period in July . If yours is 28, then it's 14 days and not 15 days.

Source: WHO fertility Year book.