1.02 The Birds and the Bees

June 23, 2017

Today, we are going to discuss the birds and the bees, or, more accurately where do babies come from?

 

 

 

Men and women pursue two very different, but complimentary strategies for reproduction. The average ejaculate contains about 250million sperm – which means that a normal male produces around a thousand sperm per second: which makes men the ultimate multitaskers.

Women on the other hand, produce eggs only once, and run out of this reserve over the period of a lifetime. Women peak at about 7 million eggs when they are still in the womb. From that point, there is an inevitable loss of eggs, regardless of whether a woman is having periods, is on the pill or is ovulation. By the time a baby girl is born, that number has already dropped to 3 million and by the time she goes through puberty that number has reduced to 1 million. By the onset of menopause, there will no viable eggs left in the ovaries. So, over her lifetime, a woman will ovulate less than 500 eggs or what your average male produces in a second.  So while men go for quantity, women for for quality. Unlike sperm, which is really almost like a missile with a DNA payload, an egg is a beautiful, complex structure, that has the capacity to repair the sperm and to allow the embryo to grow in the first few days.

Every month, a woman will produce one egg and release this, in the process known as ovulation. This egg is in fact part of a group of eggs that have started growing over 150 days prior to this point. This group of eggs has been selected by a process that we do not understand after a period of lying in wait for years to decades. 

The egg grows in a structure called a follicle, that we can see on our ultrasound scans every time we do a scan. The follicle grows until ovulation, which usually occurs around the middle of a woman’s cycle. We count cyle from day 1 of the period and the first day of your period is the first day that you have full flow – so if the period comes before lunch, we call that the first day, Don’t count any spotting. So if you have a 28 day cycle, ovulation is likely around day 14. Once the egg has been released in ovulation, it is actually only able to be fertilised for a few hours, so sperm have to be around to meet the egg.

After sperm have been ejaculated, or sometimes placed inside of the vagina, the sperm have to travel through the cervix, across the endometrial canal to the tube.

You can see the opening of the tube in this video.

In this video, you see the uterine tube and ovary on the right. After the egg has been picked up  by the tube, the sperm enters the egg in the process of fertilization. This is a lock and key mechanism, where the sperm needs to have the right key to fit the egg’s lock. The embryo then travels up the canal to the uterine cavity.

The embryo in fact spends the first 5 days in the tube, until it reaches the endometrial cavity. In that time it undergoes an incredible transition from a single cell to several hundred cells in the blastocyst. Again, implantaion is a lock and key mechanism. After finding the exact spot in the cavity that is appropriate for implantation, the embryos first attaches and then buries into the lining of the uterus.

In this time, the embryo forms the basic parts that will form the baby and the placenta. By the time a woman has missed her period, the chemical signs that an embryo has implanted, the hCG, will be detectable in both the urine and the blood.

So by the time you have missed your first period, your embryo has already taken residence for a couple of weeks. This is where there is some confusion arises – by the tiem you have missed your period you are already 4 weeks pregnant, because we count from the first day of the period, not ovulation (which would be more accurate. Similarly, a pregnancy is not 9 months, but 40 weeks, or 10 months. Sorry to disappoint, but you are better off knowing now!

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