ONE of the first things a man will do after being greeted with news of an unplanned pregnancy is to question whether he indeed planted the seed. Because with statistics showing that a large number of men aren’t really the fathers of children they thought were theirs, every man is afraid of wearing a jacket. Yet, day after day, men are hit with full suits, some repeatedly, as they aren’t aware of just how to calculate the details of a pregnancy. And many will take the woman’s word for it — the explanations for the extended or shortened pregnancies and the explanations of why the baby looks nothing like them, just on the woman’s say so.

So how can you know for sure the baby’s yours? It’s all a matter of mathematical reasoning, an exact science, explained below by gynaecologist and obstetrician at the University Hospital of the West Indies Dr Horace Fletcher.

1. She says the doctor did an examination and a urine test and determined she was five weeks pregnant. Is this possible?

Yes this is possible. Based on history of her last menstrual period (LMP) and the examination of the cervix’s bluish tinge and the positive pregnancy test we can estimate the number of weeks of amenorrhoea (number of weeks there has been no period). This is actually three weeks after she got pregnant. This causes some confusion because we check the weeks from the LMP and not from when she had s3x or ovulated, which is mid-cycle. So if a woman is told she is five weeks pregnant she is actually technically three weeks pregnant (three weeks from the conception date). If she had s3x with a man five weeks before and another man three weeks before, at five weeks the pregnancy is most likely for the one she had s3x with three weeks before. We also have to assume she has a regular cycle, between 21 and 35 days, and she was not recently taking any hormonal contraceptives which can change the cycle.

She says she had her last period December 12. How will I know when exactly she got pregnant?

In almost all women with the 28-day cycle, ovulation occurs 14 days into the cycle — that is 14 days from the start of the period. Ovulation refers to that time when the ovary releases an egg for fertilisation. If the first day of her last period was December 12, and she has a 28-day cycle, she would have been a little bit fertile on December 22; fertile on December 23; very fertile on December 24; and would have ovulated on December 25. She could have got pregnant on any one of those days. If her cycle is only 21 days, then from menstruation to ovulation is seven days and if her cycle is longer than 28 days add the number of days it is longer to the first half of the cycle (from menstruation to ovulation).

3. How soon can a doctor detect a pregnancy?

We can usually detect a pregnancy when a woman just misses a period, or two weeks after she got pregnant.

4. She said her last period began on January 1. How do I calculate due date/weeks of pregnancy based on her last period and conception date?

We calculate dates from the LMP as described above. We calculate expected date of delivery based on many years of experience of health care workers to find out approximately when the baby will be born, add seven days and one year and subtract three months. So LMP is 1/1/2012, add seven days and one year — 8/1/2013 — and subtract three months. The estimated date of delivery would be 8/10/2012. Babies can come normally two weeks before or after the estimated date.

5. How soon can a urine test/blood test detect pregnancy?

Usually both are positive as soon as a period is missed.

6. She has been sleeping around but tells me she knows for sure I am the father, how can she tell?

She can have a fair idea if you did not use a condom! But you can have a very good idea if you were the only one who slept with her during ovulation time — that is mid-cycle. If you do not want your name called, use a condom from start to finish and dispose of it yourself. There have been stories of women getting pregnant from semen in cond0ms they used to impregnate themselves. If there is still doubt there is still DNA testing .

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